Illumination device and projection display device using same

ABSTRACT

An illumination device comprises a light source that includes a solid-state light source whose peak wavelength is set in the red wavelength band, a light source that includes a solid-state light source whose peak wavelength is set in the green wavelength band, a light source that includes a solid-state light source whose peak wavelength is set in the blue wavelength band, and a color synthesis optical element that combines P-polarized colored light incident from one light source and S-polarized colored light incident from the other two light sources. One light source includes at least one solid-state light source whose peak wavelength is set in the wavelength band of the color corresponding to one of the other light sources.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/498,532, filed on Mar. 27, 2012, which is a national stage ofInternational Application No. PCT/JP2010/065601, filed on Sep. 10, 2010,which claims priority from Japanese Patent Application No. 2009-222671,filed on Sep. 28, 2009, the contents of all of which are incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a projection-type display device ofwhich a projector is representative, and more particularly relates to anillumination device that generates an illumination light in which lightof a plurality colors is synthesized and to a projection-type displaydevice that uses this illumination device.

BACKGROUND ART

A projection-type display device includes an illumination device, adisplay element that is illuminated by the illumination light from theillumination device, and a projection lens that magnifies and projectsthe image displayed on the display element onto a screen. Theillumination device is made up of a white light source, and a colorwheel in which red (R), green (G), and blue (B) color filters arearranged in a disk form. The light from the white light source isentered onto the color wheel that rotates at high speed, whereby anillumination light is obtained in which color switches in a timesequence.

In the projection-type display device that is provided with theabove-described illumination device, a full-color image can be displayedon a screen according to the principles of successive additive colormixture by displaying images of these color components on displayelements in synchronization with the switching of the illuminationlight. This type of projection-type display device is referred to as afield-sequential or time-division projection-type display device.

A high-luminance light source such as a high-pressure mercury lamp isused as the white light source. However, a discharge lamp such as ahigh-pressure mercury lamp, while having high luminance, gives rise tothe problems described hereinbelow in an illumination device that iscombined with a color wheel or in a projection-type display device thatuses such an illumination device.

Such a lamp is inconvenient to use because a lengthy time is requiredfrom being turned on until reaching a steady state of brightness, andfurther, after being turned off, a waiting period is also required forsufficient cooling before the lamp can be relighted.

When colored light is obtained from a white light source by a colorwheel, the light utilization efficiency is extremely poor because, forexample, blue and green light cannot be used during the time interval inwhich light passes through the red color filter to give redillumination.

In addition, two colors are mixed when light passes through the boundaryportions of the color filters. As a result, the color purity of lightthat has passed through the boundary portions is degraded, or the lightutilization efficiency drops because this light is not used in theprojection display.

Still further, the time or order of switching colors is fixed by thecolor wheel that is employed.

In addition to the above, there is the problem that mechanical partsthat causes the color wheel to rotate at high speed and sensors andelectronic circuits that control the stability of rotation are required,leading to a corresponding increase in the cost of the device. Stillfurther, there is the problem of noise during high-speed rotation.

Recent years have seen the development of higher luminance of lightsources such as light-emitting diodes (LEDs) and semiconductor lasers(LDs) that are referred to as semiconductor light sources or solid-statelight sources. The light emitted from these semiconductor light sourceshas a narrower spectral width than the light of white light sources ofwhich discharge lamps such as high-pressure mercury lamps arerepresentative and have the feature of enabling higher colorreproducibility to be obtained without the use of color filters.

If an illumination device is configured using, for example, red (R),green (G), and blue (B) LEDs and a color synthesis optical element inplace of a white light source and color wheel and the LEDs of each colorare successively lighted, an illumination device is obtained in whichcolors are switched in a time sequence.

In contrast to a discharge lamp in which time is required for brightnessto reach a steady state of brightness after lighting, a semiconductorlight source such as an LED obtains an illumination light as well as aprojected image that is bright immediately after lighting, and moreover,requires no waiting time for cooling before relighting. As a result,using a semiconductor light source as the light source of aprojection-type display device improves user convenience.

In addition, an LED has a longer service life than a discharge lamp andis superior from an environmental standpoint because mercury is notused. The utilization efficiency of light is high because blue and greenLEDs are extinguished when red is irradiated by an LED, and lower powerconsumption can thus be realized. Still further, installing a dimmingfunction that controls the amount of current of the LEDs enables precisepower saving according to conditions.

Because each of the red, green, and blue LEDs can be separatelycontrolled, the time and order of color switching can be controlledelectronically, colors can be switched freely, and moreover,synchronization with the display elements can be achieved with highaccuracy. Because colors can be switched at high speed, color break-upthat was problematic in the field-sequential mode color display can bemarkedly reduced. The ability to obtain not only R-G-B illuminationlight but also C (cyan), M (magenta), Y (yellow), as well as W (white)illumination light also enables display by a display mode thatprioritizes brightness by displaying these color images. Still further,the problems of deterioration of rotation mechanism parts or noisecaused by high-speed rotation do not arise.

Due to these many advantages afforded by LEDs, an illumination devicethat uses, for example, LEDs and a color synthesis optical element in aprojection-type display device is highly anticipated.

However, emission light of a sufficient brightness cannot currently beobtained from a single LED. Thus, in order to realize higher luminance,various techniques for combining a plurality of colors have beenproposed. For example, Patent Documents 1-3 disclose light sourcedevices that combine luminous flux from a plurality of LEDs havingdifferent peak wavelengths by means of dichroic mirrors or dichroicprisms. These light source devices are of a mode in which differences inwavelength are used to synthesize colored light by dichroic mirrors.

Alternatively, light source devices are disclosed in Patent Documents 4and 5 in which at least one of three light sources is of a configurationin which a plurality of light sources having different peak wavelengthsare disposed in array form. This is a mode of spatially synthesizingcolored light.

Yet another mode of synthesizing colored light is a technique that usespolarization. For example, an illumination device is disclosed in PatentDocument 6 in which light from two light sources that emit light havingrandom polarization directions is converted to linearly polarized lighthaving polarization directions that are mutually orthogonal and is thensynthesized by a polarization beam splitter.

As a related invention, Patent Document 7 discloses a light sourcedevice in which light of each color is arranged in a specificpolarization direction in advance and then synthesized by a dichroicprism. Still further, a projection-type display device is disclosed inPatent Document 8 in which the polarization direction of incident lightis selected while taking into consideration the incident angledependency when colors are synthesized by a dichroic prism.

The color synthesis optical element that is used in the light sourcedevice described in Patent Document 8 includes blue-reflectingmultilayer film and red-reflecting multilayer film. FIG. 1A shows thespectral reflectance characteristic of the blue-reflecting multilayerfilm, and FIG. 1B shows the spectral reflectance characteristic of thered-reflecting multilayer film.

As shown in FIG. 1A, the cutoff wavelength of S-polarized light of theblue-reflecting multilayer film is at least 510 nm but no greater than540 nm. As shown in FIG. 1B, on the other hand, the cutoff wavelength ofS-polarized light of the red reflecting multilayer film is at least 540nm but no greater than 560 nm.

The light (P-polarized light) from a green light valve (display element)is entered into the blue-reflecting multilayer film and thered-reflecting multilayer film, and light (S-polarized light) from redand blue light valves (display elements) is entered into blue-reflectingmultilayer film and red-reflecting multilayer film.

LITERATURE OF THE PRIOR ART Patent Documents

-   Patent Document 1: Japanese Unexamined Patent Application    Publication No. 2001-042431 (FIG. 1)-   Patent Document 2: Japanese Unexamined Patent Application    Publication No. 2005-321524 (FIG. 1)-   Patent Document 3: Japanese Unexamined Patent Application    Publication No. 2004-070018 (FIG. 5)-   Patent Document 4: Japanese Unexamined Patent Application    Publication No. 2004-325630 (FIG. 1)-   Patent Document 5: Japanese Unexamined Patent Application    Publication No. 2005-189277 (FIG. 1)-   Patent Document 6: Japanese Unexamined Patent Application    Publication No. 2006-337609 (FIG. 1)-   Patent Document 7: Japanese Unexamined Patent Application    Publication No. 2000-056410 (FIG. 7)-   Patent Document 8: Japanese Unexamined Patent Application    Publication No. H01-302385 (FIG. 1)

SUMMARY OF THE INVENTION

A projection-type display device that uses LEDs as light sources of eachof the colors red, green, and blue and that synthesizes the coloredlight from the LEDs of each color to obtain white light has the problemsas described below.

In the projection optics, there is also the constraint of etendue thatis determined by the area of the light source and the angle ofdivergence. If the value that is the product of the area of the lightsource and the angle of divergence is not made less than or equal to thevalue of the product of the area of a display element and the acceptanceangle (solid angle) that is determined by the f-number of the projectionlens, the light from the light source is not used as projection light.In other words, in the projection optics, there are constraintsregarding the area of a semiconductor chip of an LED or the number ofLEDs, and moreover, there is a constraint regarding the angular spreadof the illumination light. Essentially, brightness cannot be improvedeven if a multiplicity of LEDs greater than or equal to the numberdetermined by the constraint of etendue is aligned in an array.

Still further, because the optical output characteristic of LEDs differsfor each of the colors of red, green, and blue, the optical output ofthe LEDs of other colors must be restrained to accord with the opticaloutput of the LEDs of the color having the lowest performance. As aresult, the maximum optical output performance of LEDs of other colorscannot be realized.

Under the conditions of the constraints of etendue, an illuminationdevice that can exhibit the maximum optical output performance of theLEDs of each color and that can obtain white light having superior whitebalance is difficult to realize even by a combination of the technologydisclosed in Patent Documents 1-8.

For example, even if a device in which the polarization dependence of adichroic mirror is taken into consideration in selecting thepolarization direction of incident light (Patent Document 8) is used ina device that, taking incident angle dependency and polarizationdependency into consideration, uses polarization to synthesize light(Patent Document 6) or in a device that aligns the light of each colorin a specific polarization direction in advance and then irradiates thelight into a dichroic mirror (Patent Document 7), the light of a colorthat is inadequate at the LED light source cannot be augmented, andconsequently, the constraints of etendue cannot be canceled and theoptical output performance of all LEDs cannot be displayed at theirmaximum. Here, the incident angle dependency is the shifting of cutoffwavelengths towards shorter wavelengths or longer wavelengths from theset values according to the angle of incidence to the dichroic mirror.Polarization dependency is the difference in cutoff wavelengths betweenP-polarized light and S-polarized light.

It is therefore an object of the present invention to provide anillumination device that can obtain white light having superior whitebalance and that can display the optical output performance of an LED ata maximum and thus solve the above-described problems, and to provide aprojection-type display device that uses such an illumination device.

The illumination device of the present invention for achieving theabove-described objects includes:

a first light source that includes a solid-state light source whose peakwavelength is set in the red wavelength band;

a second light source that includes a solid-state light source whosepeak wavelength is set in the green wavelength band;

a third light source that includes a solid-state light source whose peakwavelength is set in the blue wavelength band; and

a color synthesis optical element that synthesizes colored light of afirst polarization that is entered from the above-described second lightsource and colored light of a second polarization whose polarizationstate differs from that of the above-described first polarization thatis entered from the above-described first and third light sources;

wherein any one of the above-described first to third light sourcesfurther includes at least one solid-state light source whose peakwavelength is set in a specific wavelength band that is the wavelengthband of the color of the solid-state light source that is used in one ofthe remaining two light sources.

The projection-type display device of the present invention includes:

the above-described illumination device;

a display element into which light from the above-described illuminationdevice is entered;

projection optics that project an image displayed by the above-describeddisplay elements; and

control means that both displays images that accord with an input videosignal on the above-described display element for each color componentthat corresponds to the three primary colors of light and that controlsthe lighting of the above-described first to third light sources thatmake up the above-described illumination device in synchronization withthe timing of the image display of each color component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graph showing the spectral reflectance characteristic ofblue-reflecting multilayer film of the color synthesis optical elementdescribed in Patent Document 8.

FIG. 1B is a graph showing the spectral reflectance characteristic of ared-reflecting multilayer film of the color synthesis optical elementdescribed in Patent Document 8.

FIG. 2 is a perspective view showing the configuration of theillumination device that is the first exemplary embodiment of thepresent invention.

FIG. 3A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of the firstdichroic mirror of the color synthesis optical element that forms partof the illumination device shown in FIG. 2.

FIG. 3B is a graph showing the spectral reflectance characteristic withrespect to P-polarized light and S-polarized light of the first dichroicmirror of the color synthesis optical element that forms part of theillumination device shown in FIG. 2.

FIG. 4A is a graph showing the spectral transmittance characteristic ofthe second dichroic mirror of the color synthesis optical element thatforms part of the illumination device shown in FIG. 2.

FIG. 4B is a graph showing the spectral reflectance characteristic ofthe second dichroic mirror of the color synthesis optical element thatforms part of the illumination device shown in FIG. 2.

FIG. 5 is a block diagram showing the configuration of the light sourcethat is used in the illumination device shown in FIG. 2.

FIG. 6A is a schematic view showing the configuration of the red LEDmodule that is used as a light source of the illumination device shownin FIG. 2.

FIG. 6B is a schematic view showing the configuration of the green LEDmodule that is used as a light source of the illumination device shownin FIG. 2.

FIG. 6C is a schematic view showing the configuration of the blue LEDmodule that is used as a light source of the illumination device shownin FIG. 2.

FIG. 7 is a schematic view showing an example of optical paths whencolored light is synthesized using the illumination device shown in FIG.2.

FIG. 8A is a graph showing the relation between the emission spectrum ofa green LED light source and the spectral transmittance characteristicwith respect to P-polarized light of the first dichroic mirror of thecolor synthesis optical element that forms part of the illuminationdevice shown in FIG. 2.

FIG. 8B is a graph showing the relation between the emission spectrumsof each of red, green, and blue LED light sources and the spectralreflectance characteristic with respect to S-polarized light of thefirst dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 2.

FIG. 9A is a graph showing the relation between the emission spectrum ofa green LED light source and the spectral transmittance characteristicwith respect to P-polarized light of the second dichroic mirror of thecolor synthesis optical element that forms part of the illuminationdevice shown in FIG. 2.

FIG. 9B is a graph showing the relation between the emission spectrumsof each of the red, green, and blue LED light sources and the spectralreflectance characteristic with respect to S-polarized light of thesecond dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 2.

FIG. 10 is a schematic view showing another example of optical pathswhen colors are synthesized using the illumination device shown in FIG.2.

FIG. 11A is a graph showing the relation between the emission spectrumsof each of green and red LED light sources and the spectraltransmittance characteristic with respect to P-polarized light of thefirst dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 10.

FIG. 11B is a graph showing the relation between the emission spectrumsof each of red, green and blue LED light sources and the spectralreflectance characteristic with respect to S-polarized light of thefirst dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 10.

FIG. 12A is a graph showing the relation between the emission spectrumsof each of green and red LED light sources and the spectraltransmittance characteristic with respect to P-polarized light of thesecond dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 10.

FIG. 12B is a graph showing the relation between the emission spectrumsof each of red, green, and blue LED light sources and the spectralreflectance characteristic with respect to S-polarized light of thesecond dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 10.

FIG. 13 is a perspective view showing the configuration of theillumination device that is the second exemplary embodiment of thepresent invention.

FIG. 14A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of the firstdichroic mirror of the color synthesis optical element that forms partof the illumination device shown in FIG. 13.

FIG. 14B is a graph showing the spectral reflectance characteristic withrespect to P-polarized light and S-polarized light of the first dichroicmirror of the color synthesis optical element that forms part of theillumination device shown in FIG. 13.

FIG. 15A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of the seconddichroic mirror of the color synthesis optical element that forms partof the illumination device shown in FIG. 13.

FIG. 15B is a graph showing the spectral reflectance characteristic withrespect to P-polarized light and S-polarized light of the seconddichroic mirror of the color synthesis optical element that forms partof the illumination device shown in FIG. 13.

FIG. 16 is a schematic view showing an example of the optical paths whencolored light is synthesized using the illumination device shown in FIG.13.

FIG. 17A is a graph showing the relation between the emission spectrumof a green LED light source and the spectral transmittancecharacteristic with respect to P-polarized light of the first dichroicmirror of the color synthesis optical element that forms part of theillumination device shown in FIG. 13.

FIG. 17B is a graph showing the relation between the emission spectrumsof each of the red, green, and blue LED light sources and the spectralreflectance characteristic with respect to S-polarized light of thefirst dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 13.

FIG. 18A is a graph showing the relation between the emission spectrumof a green LED light source and the spectral transmittancecharacteristic with respect to P-polarized light of the second dichroicmirror of the color synthesis optical element that forms part of theillumination device shown in FIG. 13.

FIG. 18B is a graph showing the relation between the emission spectrumsof each of red, green, and blue LED light sources and the spectralreflectance characteristic with respect to S-polarized light of thesecond dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 13.

FIG. 19 is a perspective view showing the configuration of theillumination device that is the third exemplary embodiment of thepresent invention.

FIG. 20 is a perspective view showing the configuration of theillumination device that is the fourth exemplary embodiment of thepresent invention.

FIG. 21A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of the firstdichroic mirror of the color synthesis optical element that forms partof the illumination device shown in FIG. 20.

FIG. 21B is a graph showing the spectral reflectance characteristic withrespect to P-polarized light and S-polarized light of the first dichroicmirror of the color synthesis optical element that forms part of theillumination device shown in FIG. 20.

FIG. 22A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of the seconddichroic mirror of the color synthesis optical element that forms partof the illumination device shown in FIG. 20.

FIG. 22B is a graph showing the spectral reflectance characteristic withrespect to P-polarized light and S-polarized light of the seconddichroic mirror of the color synthesis optical element that forms partof the illumination device shown in FIG. 20.

FIG. 23 is a schematic view showing an example of the optical paths whencolors are synthesized using the illumination device shown in FIG. 20.

FIG. 24A is a graph showing the relation between the emission spectrumsof each of blue and green LED light sources and the spectraltransmittance characteristic with respect to P-polarized light of thefirst dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 20.

FIG. 24B is a graph showing the relation between the emission spectrumsof each of green and red LED light sources and the spectral reflectancecharacteristic with respect to S-polarized light of the first dichroicmirror of the color synthesis optical element that forms part of theillumination device shown in FIG. 20.

FIG. 25A is a graph showing the relation between the emission spectrumsof each of blue and green LED light sources and the spectraltransmittance characteristic with respect to P-polarized light of thesecond dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 20.

FIG. 25B is a graph showing the relation between the emission spectrumsof each of green and red LED light sources and the spectral reflectancecharacteristic with respect to S-polarized light of the second dichroicmirror of the color synthesis optical element that forms part of theillumination device shown in FIG. 20.

FIG. 26 is a perspective view showing the configuration of theillumination device that is the fifth exemplary embodiment of thepresent invention.

FIG. 27A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of the firstdichroic mirror of the color synthesis optical element that forms partof the illumination device shown in FIG. 26.

FIG. 27B is a graph showing the spectral reflectance characteristic withrespect to P-polarized light and S-polarized light of the first dichroicmirror of the color synthesis optical element that forms part of theillumination device shown in FIG. 26.

FIG. 28A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of the seconddichroic mirror of the color synthesis optical element that forms partof the illumination device shown in FIG. 26.

FIG. 28B is a graph showing the spectral reflectance characteristic withrespect to P-polarized light and S-polarized light of the seconddichroic mirror of the color synthesis optical element that forms partof the illumination device shown in FIG. 26.

FIG. 29 is a schematic view showing an example of the optical paths whencolors are synthesized using the illumination device shown in FIG. 26.

FIG. 30A is a graph showing the relation between the emission spectrumsof each of green and red LED light sources and the spectraltransmittance characteristic with respect to P-polarized light of thefirst dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 26.

FIG. 30B is a graph showing the relation between the emission spectrumsof each of blue and green LED light sources and the spectral reflectancecharacteristic with respect to S-polarized light of the first dichroicmirror of the color synthesis optical element that forms part of theillumination device shown in FIG. 26.

FIG. 31A is a graph showing the relation between the emission spectrumsof each of green and red LED light sources and the spectraltransmittance characteristic with respect to P-polarized light of thesecond dichroic mirror of the color synthesis optical element that formspart of the illumination device shown in FIG. 26.

FIG. 31B is a graph showing the relation between the emission spectrumsof each of blue and green LED light sources and the spectral reflectancecharacteristic with respect to S-polarized light of the second dichroicmirror of the color synthesis optical element that forms part of theillumination device shown in FIG. 26.

FIG. 32 is a block diagram showing the configuration of theprojection-type display device that is the sixth exemplary embodiment ofthe present invention.

FIG. 33 is a block diagram for describing the control means of theprojection-type display device shown in FIG. 32.

FIG. 34 is a block diagram showing the configuration of theprojection-type display device that is the seventh exemplary embodimentof the present invention.

EXPLANATION OF REFERENCE NUMBERS

-   1 color synthesis optical element-   1 a-1 d right angle prisms-   2 a first dichroic mirror-   2 b second dichroic mirror-   3 a-3 c light sources

BEST MODE FOR CARRYING OUT THE INVENTION

Exemplary embodiments of the present invention are next described withreference to the accompanying drawings.

First Exemplary Embodiment

FIG. 2 is a perspective view showing the configuration of theillumination device that is the first exemplary embodiment of thepresent invention.

Referring to FIG. 2, the illumination device includes color synthesisoptical element 1 and three light sources 3 a-3 c.

Color synthesis optical element 1 is a cross dichroic prism made up offour right angle prisms 1 a-1 d in which the surfaces that form rightangles are joined together. A uniform first plane is formed by thejoined surfaces of right angle prisms 1 a and 1 d and the joinedsurfaces of right angle prisms 1 b and 1 c, and first dichroic mirror 2a composed of a dielectric multilayer film is formed on this firstplane. A uniform second plane that intersects the first plane is formedby the joined surfaces of right angle prisms 1 a and 1 b and the joinedsurfaces of right angle prisms 1 c and 1 d, and second dichroic mirror 2b composed of dielectric multilayer film is formed on this second plane.In other words, first dichroic mirror 2 a and second dichroic mirror 2 bare formed such that their film surfaces intersect each other.

Light is entered from three surfaces (each of the surfaces of rightangle prisms 1 a, 1 c, and 1 d) of the four side surfaces of colorsynthesis optical element 1 and colors are synthesized. The oneremaining side surface is the exit surface of the synthesized light.

Light source 3 a supplies red light (S-polarized light). Light source 3b supplies green light (P-polarized light). Light source 3 c suppliesgreen and blue light (S-polarized light). Here, red, green, and bluecorrespond to the three primary colors of light.

The S-polarized light (red) from light source 3 a is entered into colorsynthesis optical element 1 from the incident surface of right angleprism 1 c. The P-polarized light (green) from light source 3 b isentered into color synthesis optical element 1 from the incident surfaceof right angle prism 1 d. The S-polarized light (green+blue) from lightsource 3 c is entered into color synthesis optical element 1 from theincident surface of right angle prism 1 a.

In color synthesis optical element 1, the S-polarized light (red),P-polarized light (green), and S-polarized light (green+blue) from eachincident surface are synthesized by first dichroic mirror 2 a and seconddichroic mirror 2 b.

FIG. 3A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of firstdichroic mirror 2 a. FIG. 3B is a graph showing the spectral reflectancecharacteristic with respect to P-polarized light and S-polarized lightof the first dichroic mirror 2 a.

The cutoff wavelength is defined as the wavelength at which thetransmittance or reflectance is 50%. The cutoff wavelength of firstdichroic mirror 2 a with respect to incident P-polarized light is 400nm. In this case, first dichroic mirror 2 a largely transmits and doesnot reflect P-polarized light having a wavelength of 400 nm or more. Onthe other hand, the cutoff wavelength of first dichroic mirror 2 a withrespect to incident S-polarized light is 580 nm. In this case, firstdichroic mirror 2 a largely transmits and does not reflect S-polarizedlight having a wavelength of 580 nm or more. In addition, first dichroicmirror 2 a largely reflects and does not transmit S-polarized lighthaving a wavelength shorter than 580 nm.

If the characteristics of first dichroic mirror 2 a are expressed by itsaction upon colored light, with respect to blue and green light, firstdichroic mirror 2 a transmits P-polarized light and reflects S-polarizedlight. In other words, first dichroic mirror 2 a also acts as apolarization beam splitter with respect to blue and green light. Inaddition, with respect to red light, first dichroic mirror 2 a transmitsand does not act in any way upon either of P-polarized light andS-polarized light.

FIG. 4A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of seconddichroic mirror 2 b. FIG. 4B is a graph showing the spectral reflectancecharacteristic with respect to P-polarized light and S-polarized lightof second dichroic mirror 2 b.

The cutoff wavelength of second dichroic mirror 2 b with respect toincident P-polarized light is 700 nm. In this case, second dichroicmirror 2 b largely transmits and does not reflect P-polarized lighthaving a wavelength of 700 nm or less. On the other hand, the cutoffwavelength of second dichroic mirror 2 b with respect to incidentS-polarized light is 580 nm. In this case, second dichroic mirror 2 blargely reflects and does not transmit S-polarized light having awavelength of 580 nm or more. In addition, second dichroic mirror 2 blargely transmits and does not reflect S-polarized light having awavelength shorter than 580 nm.

If the characteristics of second dichroic mirror 2 b are expressed asits action upon colored light, with respect to blue and green light,second dichroic mirror 2 b transmits and does not act in any way uponeither of P-polarized light and S-polarized light. In addition, withrespect to red light, second dichroic mirror 2 b transmits P-polarizedlight and reflects S-polarized light. In other words, second dichroicmirror 2 b also acts as a polarization beam splitter with respect to redlight.

FIG. 5 is a block diagram showing the basic configuration of a lightsource that is used as light sources 3 a-3 c. Referring to FIG. 5, thelight source includes LED module 50 in which an LED that islight-emitting unit 51 is mounted on a substrate. The substrate alsoprovides the function of radiator plate and a heat sink (not shown) isattached. A forced cooling apparatus is further provided on LED module50 and temperature control is carried out such that the light-emittingcharacteristic of the LED is stabilized.

When the switch of the illumination device is turned on, drive circuit53 supplies drive current to light-emitting unit (LED) 51. When currentflows in light-emitting unit (LED) 51, light-emitting unit (LED) 51emits light. The light from light-emitting unit (LED) 51 is condensed bycondensing optics 52. The luminous flux from condensing optics 52 isentered into color synthesis optical element 1.

Although an optical element in the form of a lens is used as thecondensing optics in FIG. 5, a reflective optical element such as areflector may also be used. In addition, a fly-eye lens or glass rod maybe used as an integrator for illuminating the display element uniformly.Still further, in order to efficiently obtain polarization components,optics may be used for re-using one polarization component, such aspolarization conversion optics that use a polarization beam splitter anda half-wave plate. Of course, light-emitting unit 51 of LED module 50may be a light source that emits polarized light, or a configuration maybe adopted in which a polarization conversion function is provided inlight-emitting unit 51 such that polarized light is emitted fromlight-emitting unit 51. Any form can be configured through the freecombination of known technology.

An actual configuration of the LED module of light sources 3 a-3 c isnext described.

FIG. 6A is a schematic view showing the configuration of the red LEDmodule that is used in light source 3 a. Referring to FIG. 6A, red LEDmodule 60 includes light-emitting unit 61 that is composed of four LEDchips 61 a-61 d. All of LED chips 61 a-61 d are red LEDs having a peakwavelength of 630 nm and all have substantially identical chip areas.

FIG. 6B is a schematic view showing the configuration of the green LEDmodule that is used in light source 3 b. Referring to FIG. 6B, green LEDmodule 70 includes light-emitting unit 71 composed of four LED chips 71a-71 d. All of LED chips 71 a-71 d are green LEDs having a peakwavelength of 520 nm and all have substantially identical chip areas.

FIG. 6C is a schematic view showing the configuration of the blue LEDmodule that is used in light source 3 c. Referring to FIG. 6C, blue LEDmodule 80 includes light-emitting unit 81 composed of four LED chips 81a-81 d. All of LED chips 81 a-81 c are blue LEDs having a peakwavelength of 460 nm. LED chip 81 d is a green LED having a peakwavelength of 520 nm. The chip areas of LED chips 81 a-81 d aresubstantially identical.

The emission spectrums of the red, green, and blue LEDs that make up theabove-described LED chips 61 a-61 d, 71 a-71 d, and 81 a-81 d aresimilar to the emission spectrums shown in FIG. 8B, to be described.

The areas of each of light-emitting units 61, 71, and 81 are basicallydetermined by the area of the display elements and the f-number of theprojection lens based on the previously described constraints ofetendue, but when determining the areas, the positioning margins duringmanufacturing and the uniformity of the illuminance distribution of theillumination light are taken into consideration.

In red LED module 60, green LED module 70, and blue LED module 80, thelight-emission characteristics vary with respect to current of the LEDchips that make up the light-emitting units, and the amount of currentto the LED chips is controlled by the drive circuit shown in FIG. 5 inaccordance with this light-emission characteristic.

In addition, the characteristics of each color of the LEDs during rateddrive are as follows. The chromaticity of the red LEDs is given as(0.700, 0.300) on xy chromaticity coordinates and the emitted luminousflux is 455 lm per chip. The chromaticity of the green LEDs is given as(0.195, 0.700) on xy chromaticity coordinates and the emitted luminousflux is 1000 lm per chip. The chromaticity of the blue LEDs is (0.140,0.046) on xy chromaticity coordinates and the emitted luminous flux is133 lm per chip.

The setting of the cutoff wavelength with respect to S-polarized lightof first dichroic mirror 2 a and second dichroic mirror 2 b to theyellow band of 580 nm differs greatly from the spectral characteristicsof the dichroic prism disclosed in Patent Document 8 (see FIGS. 1A and1B). Due to this point of difference, the light of a color that isinadequate can be augmented within the constraints of etendue to obtaingood white balance and the optical output characteristic of LED lightsources can be displayed at a maximum. These features will be describedin detail hereinbelow.

FIG. 7 is a plan view for describing the optical paths when coloredlight is synthesized using the illumination device shown in FIG. 2.

Of the four side surfaces of color synthesis optical element 1, threesurfaces are incident surfaces and the colored light that is enteredfrom these incident surfaces is synthesized by first dichroic mirror 2 aand second dichroic mirror 2 b. The one remaining surface is the exitsurface, and the synthesized colored light is emitted from this exitsurface.

In FIG. 7, the lines represented by solid lines with arrows each showrepresentative directions of progression of incident luminous flux, butthis is not intended to indicate that the incident rays are only thelines represented by these solid lines with arrows. The incident lightis luminous flux having a cross-sectional area no greater than theincident surfaces of color synthesis optical element 1 and includespositions other than the lines represented by the solid lines witharrows and further includes angular components.

Light source 3 a emits red S-polarized light. The red S-polarized lightfrom light source 3 a is entered into color synthesis optical element 1from the incident surface of right angle prism 1 c (in FIG. 7, thesurface located on the lower side of the figure).

First dichroic mirror 2 a does not act in any way upon red S-polarizedlight and the red S-polarized light therefore passes through firstdichroic mirror 2 a without change. On the other hand, second dichroicmirror 2 b reflects all red S-polarized light. As a result, the luminousflux of red S-polarized light is bent 90 degrees at second dichroicmirror 2 b and then exited from the exit surface of right angle prism 1b, as shown in FIG. 7.

Light source 3 b emits green P-polarized light. The green P-polarizedlight from light source 3 b is entered into color synthesis opticalelement 1 from the incident surface of right angle prism 1 d (in FIG. 7,the surface located on the left side of the figure).

Neither first dichroic mirror 2 a nor second dichroic mirror 2 b act inany way upon green P-polarized light, and the green P-polarized lighttherefore passes through first dichroic mirror 2 a and second dichroicmirror 2 b without alteration. The green P-polarized light that haspassed through first dichroic mirror 2 a and second dichroic mirror 2 bis exited from the exit surface of right angle prism 1 b.

Light source 3 c emits blue and green S-polarized light. The blue andgreen S-polarized light from light source 3 c is entered into colorsynthesis optical element 1 from the incident surface of right angleprism 1 a (in FIG. 7, the surface located on the upper side of thefigure).

Second dichroic mirror 2 b does not act in any way upon blue and greenS-polarized light, and the blue and green S-polarized light thereforepasses through second dichroic mirror 2 b without alteration. On theother hand, first dichroic mirror 2 a reflects all blue and greenS-polarized light. As a result, the luminous flux of blue and greenS-polarized light is bent 90 degrees at first dichroic mirror 2 a andthen exited from the exit surface of right angle prism 1 b as shown inFIG. 7.

As described hereinabove, white light can be obtained in theillumination device of the present exemplary embodiment by the synthesisin first dichroic mirror 2 a and second dichroic mirror 2 b of green andblue S-polarized light entered from incident surface of right angleprism 1 a, red S-polarized light entered from the incident surface ofright angle prism 1 c, and green P-polarized light entered from theincident surface of right angle prism 1 d.

FIG. 8A is a graph showing the relation between the emission spectrum ofa green LED light source and the spectral transmittance characteristicwith respect to P-polarized light of first dichroic mirror 2 a. FIG. 8Bis a graph showing the relation between the emission spectrums of eachof red, green, and blue LED light sources and the spectral reflectancecharacteristic with respect to S-polarized light of first dichroicmirror 2 a. The peak wavelength of the red LED light source is 630 nm,the peak wavelength of the green LED light source is 520 nm, and thepeak wavelength of the blue LED light source is 460 nm.

FIG. 9A is a graph showing the relation between the emission spectrum ofa green LED light source and the spectral transmittance characteristicwith respect to P-polarized light of second dichroic mirror 2 b. FIG. 9Bis a graph showing the relation between the emission spectrums of eachof red, green, and blue LED light sources and the spectral reflectancecharacteristic with respect to S-polarized light of second dichroicmirror 2 b. The peak wavelength of the red LED light source is 630 nm,the peak wavelength of the green LED light source is 520 nm, and thepeak wavelength of the blue LED light source is 460 nm.

As can be clearly seen from FIGS. 8A and 9A, the cutoff wavelengths withrespect to green P-polarized light of first dichroic mirror 2 a andsecond dichroic mirror 2 b are sufficiently separated. Accordingly,green P-polarized light will not be reflected by these dichroic mirrors2 a and 2 b despite shift of the cutoff wavelength due to incident angledependency, and loss will therefore not occur due to incident angledependency.

In addition, as can be clearly seen from FIGS. 8B and 9B, the cutoffwavelengths of first dichroic mirror 2 a and second dichroic mirror 2 bwith respect to green S-polarized light and red S-polarized light aresufficiently separated. Accordingly, red and green S-polarized light canbe synthesized by these dichroic mirrors 2 a and 2 b with virtually noloss despite shifts in the cutoff wavelengths due to incident angledependency.

The cutoff wavelengths of first dichroic mirror 2 a and second dichroicmirror 2 b are thus set to the yellow band which is not used in colorsynthesis, whereby light that is entered at angles different fromparallel light can also be efficiently synthesized.

Typically, when semiconductor light sources such as LEDs are used aseach of red, green, and blue light sources and the red, green, and bluelight from each semiconductor light source is synthesized to obtainwhite light having superior white balance, regarding the color mixtureratios of red, green, and blue light, the blue optical output is greaterthan for other colors and the green optical output is smaller than forthe other colors. In this case, the optical output of the blue and redsemiconductor light sources is suppressed to accord with the greensemiconductor light source for which optical output is relatively small,and as a consequence, the optical output of the obtained white light issmall.

According to the illumination device of the present exemplaryembodiment, green light can be synthesized from two differentdirections. Still further, a configuration is adopted that reduces theamount of blue light for which the optical output is relatively greatand adds green light. Accordingly, the three primary colors can besynthesized at preferable color mixture ratios and white light havingsuperior white balance can be obtained. Still further, the opticaloutput of the LEDs of three colors can be exhibited at the maximumwithout limitation.

The effects of the illumination device of the invention of the presentapplication are next described.

As an example, the light-emitting unit of a blue LED module is made upof four blue LEDs, the light-emitting unit of a green LED module is madeup of four green LEDs, and the light-emitting unit of a red LED moduleis made up of four red LEDs. When luminous flux from each of the blue,green, and red LED modules of this type is synthesized, the entiresynthesized luminous flux is 6352 lm (=(455+100+133)×4).

However, the chromaticity of the above-described synthesized white is(0.299, 0.271), a value that diverges greatly toward blue-violet fromthe white chromaticity (0.313, 0.329) of standard illuminant D65. Thereason for this divergence is the relative weakness of the opticaloutput of the green LEDs and the relative strength of the optical outputof blue LEDs with respect to the light amount ratios for obtaining apreferable white.

To obtain white balance, the emitted luminous flux of green must beincreased. The emitted luminous flux can be increased by increasing thecurrent that flows to the LEDs if within the rated range. However, ifthe amount of current is increased in a state in which the emittedluminous flux from the green LEDs is 1000 lm, the LEDs are driven inexcess of their rating, in which case an increase of luminous fluxaccording to the increase of the amount of current cannot be expected.In addition, driving an LED in excess of its rating not only shortensthe service life of the LED, but in some cases may destroy the LED.

As described hereinabove, both the emitted luminous flux of the blueLEDs is suppressed from 133 lm to 80 lm and the emitted luminous flux ofthe red LEDs is suppressed from 455 lm to 364 lm to accord with theemitted luminous flux of the green LEDs. In this case, the totalluminous flux is 5776 lm, which is a 9% reduction of brightness.

In contrast, in the illumination device of the present exemplaryembodiment, blue LED module 80 is made up of three LED chips 81 a-81 cthat emit blue light and one LED chip 81 d that emits green light, asshown in FIG. 6C. In other words, in this blue LED module, the number ofblue LED chips is decreased by one compared to the previously describedblue LED module that is made up of four blue LEDs, and one LED chip thatemits green light is disposed in its place.

In addition, red LED module 60 is composed of four LED chips 61 a-61 dthat emit red light as shown in FIG. 6A, and green LED module 70 iscomposed of four LED chips 71 a-71 d that emit green light as shown inFIG. 6B. Accordingly, the number of green LED chips is the four LEDchips 71 a-71 d that are provided in green LED module 70 and one LEDchip 81 d that is provided in the blue LED module for a total of five.The number of blue LED chips becomes three, and the number of red LEDchips is four. When these red, green, and blue LED chips are all drivenat rating, a white chromaticity of (0.313, 0.329) of the standardilluminant D65 is obtained. Moreover, the total luminous flux is 7219lm, enabling a 25% improvement over the previously described 5776 lm.

According to the present exemplary embodiment as described hereinabove,an illumination device is obtained that can exhibit the optical outputperformance of LEDs at their maximum, can raise the light utilizationefficiency during color mixing, and can obtain white light havingsuperior white balance.

The illumination device of the present exemplary embodiment is notlimited to a configuration that synthesizes only green light from twodifferent directions. For example, light source 3 a may emit redS-polarized light, light source 3 b may emit green and red P-polarizedlight, and light source 3 c may emit green and blue S-polarized light asshown in FIG. 10. In this case, the green and blue S-polarized light isentered into the incident surface of right angle prism 1 a, the redS-polarized light is entered into the incident surface of right angleprism 1 c, and the green and red P-polarized light is entered into theincident surface of right angle prism 1 d.

FIG. 11A is a graph that shows the relation between the emissionspectrums of each of green and red LED light sources and the spectraltransmittance characteristic with respect to P-polarized light of firstdichroic mirror 2 a in the color synthesis optical element of theillumination device shown in FIG. 10. FIG. 11B is a graph showing therelation between the emission spectrums of each of red, green, and blueLED light sources and the spectral reflectance characteristic withrespect to S-polarized light of first dichroic mirror 2 a in the colorsynthesis optical element of the illumination device shown in FIG. 10.The peak wavelength of the red LED light source is 630 nm, the peakwavelength of the green LED light source is 520 nm, and the peakwavelength of the blue LED light source is 460 nm.

FIG. 12A is a graph showing the relation between the emission spectrumof the green LED light source and the spectral transmittancecharacteristic with respect to P-polarized light of second dichroicmirror 2 b in the color synthesis optical element of the illuminationdevice shown in FIG. 10. FIG. 12B is a graph showing the relationbetween the emission spectrums of each of red, green, and blue LED lightsources and the spectral reflectance characteristic with respect toS-polarized light of second dichroic mirror 2 b in the color synthesisoptical element in the illumination device shown in FIG. 10. The peakwavelength of the red LED light source is 630 nm, the peak wavelength ofthe green LED light source is 520 nm, and the peak wavelength of theblue LED light source is 460 nm.

As can be seen from FIGS. 11A and 12A, first dichroic mirror 2 a andsecond dichroic mirror 2 b do not act in any way upon green and redP-polarized light. As a result, the green and red P-polarized light thatis entered from the incident surface of right angle prism 1 d passesthrough each of dichroic mirrors 2 a and 2 b without alteration and isthen exited from the exit surface of right angle prism 1 b. The actionof each of dichroic mirror 2 a and 2 b upon red, green, and blueS-polarized light is as shown in FIG. 7.

The cutoff wavelengths with respect to green and red P-polarized lightof first dichroic mirror 2 a and second dichroic mirror 2 b aresufficiently separated. Accordingly, green and red P-polarized light arenot reflected by these dichroic mirrors 2 a and 2 b despite shifting ofthe cutoff wavelengths due to incident angle dependency, whereby lossdue to incident angle dependency does not occur.

In addition, as can be clearly seen from FIGS. 11B and 12B, the cutoffwavelengths with respect to green and red S-polarized light of firstdichroic mirror 2 a and second dichroic mirror 2 b are sufficientlyseparated. Accordingly, red and green S-polarized light can besynthesized in these dichroic mirrors 2 a and 2 b with virtually no lossdespite shifting of the cutoff wavelengths due to incident angledependency.

According to the illumination device shown in FIG. 10, not only cangreen light be entered from two different directions and synthesized,but red light can also be entered from two different directions andsynthesized.

In the illumination device shown in FIG. 7 or FIG. 10, light source 3 bmay be of a configuration that further emits blue P-polarized light. Inthis case, blue light can also be entered from two different directionsand synthesized.

In the illumination device of the present exemplary embodiment, thecolors among red, green, and blue light that are subjected to colormixing from two directions can be set as appropriate according todesign.

Second Exemplary Embodiment

FIG. 13 is a perspective view showing the configuration of theillumination device that is the second exemplary embodiment of thepresent invention.

Referring to FIG. 13, the illumination device includes color synthesisoptical element 11 and three light sources 13 a-13 c.

As with the first exemplary embodiment, color synthesis optical element11 is a cross dichroic prism composed of four right angle prisms 11 a-11d in which surfaces that form right angles are joined together. Firstdichroic mirror 12 a and second dichroic mirror 12 b are composed ofdielectric multilayer films formed on the joined surfaces of right angleprisms 11 a-11 d so as to intersect.

Of the four side surfaces of color synthesis optical element 11, lightis entered from three surfaces (the surfaces of right angle prisms 11 a,11 c, and 11 d) and colors are synthesized. The one remaining sidesurface is the exit surface of the synthesized light.

Light source 13 a emits red and green light (S-polarized light). Lightsource 13 b emits green light (P-polarized light). Light source 13 cemits blue light (S-polarized light). Here, red, green, and bluecorrespond to the three primary colors of light.

The S-polarized light (red+green) from light source 13 a is entered intocolor synthesis optical element 11 from the incident surface of rightangle prism 11 c. The P-polarized light (green) from light source 13 bis entered into color synthesis optical element 11 from the incidentsurface of right angle prism 11 d. The S-polarized light (blue) fromlight source 13 c is entered into color synthesis optical element 11from the incident surface of right angle prism 11 a.

In color synthesis optical element 11, the S-polarized light(red+green), P-polarized light (green), and S-polarized light (blue)from each of the incident surfaces are synthesized by first dichroicmirror 12 a and second dichroic mirror 12 b.

In the illumination device of the first exemplary embodiment, green andblue S-polarized light are entered into color synthesis optical element1 from the incident surface of right angle prism 1 a. In contrast, inthe illumination device of the present exemplary embodiment, greenS-polarized light is entered into color synthesis optical element 11together with red S-polarized light not from the incident surface ofright angle prism 11 a but from the incident surface of right angleprism 11 c that is opposite this incident surface. This is the point ofdifference between illumination device of the present exemplaryembodiment and the color synthesis optical element 1 of the firstexemplary embodiment.

FIG. 14A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of firstdichroic mirror 12 a. FIG. 14B is a graph showing the spectralreflectance characteristic with respect to P-polarized light andS-polarized light of first dichroic mirror 12 a.

The cutoff wavelength of first dichroic mirror 12 a with respect tolight entered as P-polarized light is 400 nm. In this case, firstdichroic mirror 12 a largely transmits and does not reflect P-polarizedlight having a wavelength of 400 nm or more. On the other hand, thecutoff wavelength of first dichroic mirror 12 a with respect to lightthat is entered as S-polarized light is 490 nm. In this case, firstdichroic mirror 12 a largely transmits and does not reflect S-polarizedlight having a wavelength of 490 nm or more. In addition, first dichroicmirror 12 a largely reflects and does not transmit light of S-polarizedlight having a wavelength shorter than 490 nm.

If the characteristics of first dichroic mirror 12 a are expressed byits action upon colored light, with respect to blue light, firstdichroic mirror 12 a transmits P-polarized light and reflectsS-polarized light. In other words, first dichroic mirror 12 a also actsas a polarization beam splitter upon blue light. On the other hand,first dichroic mirror 12 a does not act in any way upon green and redlight and transmits both P-polarized light and S-polarized light.

FIG. 15A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of seconddichroic mirror 12 b. FIG. 15B is a graph showing the spectralreflectance characteristic with respect to P-polarized light andS-polarized light of second dichroic mirror 12 b.

The cutoff wavelength of second dichroic mirror 12 b with respect tolight that is entered as P-polarized light is 700 nm. In this case,second dichroic mirror 12 b largely transmits and does not reflectP-polarized light having a wavelength of 700 nm or less. On the otherhand, the cutoff wavelength of second dichroic mirror 12 b with respectto light that is entered as S-polarized light is 490 nm. In this case,second dichroic mirror 12 b largely reflects and does not transmitS-polarized light having a wavelength of 490 nm or more. In addition,second dichroic mirror 12 b largely transmits and does not reflectS-polarized light having a wavelength shorter than 490 nm.

If the characteristics of second dichroic mirror 12 b are expressed byits action upon colored light, second dichroic mirror 12 b does not actin any way upon blue light and transmits both P-polarized light andS-polarized light. In addition, with respect to green and red light,second dichroic mirror 12 b transmits P-polarized light and reflectsS-polarized light. In other words, second dichroic mirror 12 b also actsas a polarization beam splitter with respect to green and red light.

The illumination device of the present exemplary embodiment differs fromthe dichroic prism that was disclosed in Patent Document 8 in that thecutoff wavelength with respect to S-polarized light of first dichroicmirror 12 a and second dichroic mirror 12 b is set to the blue-green(cyan) band of 490 nm. According to this point of difference, the lightof a color that is insufficient can be augmented within the constraintsof etendue in order to obtain good white balance and the optical outputcharacteristic of LED light sources can be exhibited to the maximum.This feature will be described in detail hereinbelow.

FIG. 16 is a plan view for describing the optical paths when coloredlight is synthesized using the illumination device shown in FIG. 13.

Of the four side surfaces of color synthesis optical element 11, threesurfaces are incident surfaces and colored light that is entered fromthese incident surfaces is synthesized by first dichroic mirror 12 a andsecond dichroic mirror 12 b. The one remaining surface is the exitsurface, and colored light that has been synthesized is exited from thisexit surface.

In FIG. 16, lines that are represented as solid lines with arrows eachshow representative directions of the progression of incident luminousflux, but this is not intended to indicate that only the linesrepresented as solid lines with arrows are the incident rays. Theincident light is luminous flux having a cross-sectional area no greaterthan the incident surfaces of color synthesis optical element 11 andincludes positions other than the lines represented by solid lines witharrows as well as rays having an angular component.

Light source 13 c emits blue S-polarized light. The blue S-polarizedlight from light source 13 c is entered into color synthesis opticalelement 11 from the incident surface of right angle prism 11 a (in FIG.16, the surface located on the upper side of the figure). Seconddichroic mirror 12 b does not act in any way upon blue S-polarized lightand the blue S-polarized light therefore passes through second dichroicmirror 12 b without alteration. On the other hand, first dichroic mirror12 a reflects all blue S-polarized light. As a result, luminous flux ofblue S-polarized light is bent 90 degrees at first dichroic mirror 12 aand is then exited from the exit surface of right angle prism 11 b.

Light source 13 b emits green P-polarized light. The green P-polarizedlight from light source 13 b is entered into color synthesis opticalelement 11 from the incident surface of right angle prism 11 d (in FIG.16, the surface located on the left side of the figure). Neither offirst dichroic mirror 12 a and second dichroic mirror 12 b acts in anyway upon green P-polarized light, and green P-polarized light thereforepasses through each of dichroic mirrors 12 a and 12 b without alterationand then is exited from the exit surface of right angle prism 11 b.

Light source 13 a emits red and green S-polarized light. The red andgreen S-polarized light from light source 13 a is entered into colorsynthesis optical element 11 from the incident surface of right angleprism 11 c (in FIG. 16, the surface located on the lower side of thefigure). First dichroic mirror 12 a does not act in any way upon greenand red S-polarized light, and the green and red S-polarized lighttherefore passes through first dichroic mirror 12 a without alteration.On the other hand, second dichroic mirror 12 b reflects all green andred S-polarized light, whereby the luminous flux of green and redS-polarized light is bent 90 degrees at first dichroic mirror 12 b andis then exited from the exit surface of right angle prism 11 b as shownin FIG. 16.

Thus, according to the illumination device of the present exemplaryembodiment, blue S-polarized light, green P-polarized light andS-polarized light, and red S-polarized light can be synthesized toobtain white light.

FIG. 17A is a graph showing the relation between the emission spectrumof a green LED light source and the spectral transmittancecharacteristic with respect to P-polarized light of first dichroicmirror 12 a. FIG. 17B is a graph showing the relation between theemission spectrums of each of red, green, and blue LED light sources andthe spectral reflectance characteristic with respect to S-polarizedlight of first dichroic mirror 12 a. The peak wavelength of the red LEDlight source is 630 nm, the peak wavelength of the green LED lightsource is 530 nm, and the peak wavelength of the blue LED light sourceis 450 nm.

FIG. 18A is a graph showing the relation between the emission spectrumof a green LED light source and the spectral transmittancecharacteristic with respect to P-polarized light of second dichroicmirror 12 b. FIG. 18B is a graph showing the relation between theemission spectrums of each of red, green, and blue LED light sources andthe spectral reflectance characteristic with respect to S-polarizedlight of second dichroic mirror 12 b. The peak wavelength of the red LEDlight source is 630 nm, the peak wavelength of the green LED lightsource is 530 nm, and the peak wavelength of the blue LED light sourceis 450 nm.

As is clear from FIGS. 17A and 18A, the cutoff wavelengths of firstdichroic mirror 12 a and second dichroic mirror 12 b with respect togreen P-polarized light are sufficiently separated. Accordingly, greenP-polarized light is not reflected by these dichroic mirrors 12 a and 12b despite shifting of the cutoff wavelengths due to incident angledependency. As a result, loss does not occur due to incident angledependency.

As is clear from FIGS. 17B and 18B, the cutoff wavelengths of firstdichroic mirror 12 a and second dichroic mirror 12 b with respect toblue S-polarized light and green S-polarized light are sufficientlyseparated. Accordingly, blue and green S-polarized light can besynthesized by these dichroic mirrors 12 a and 12 b with virtually noloss despite shifting of the cutoff wavelengths due to incident angledependency.

In this way, the cutoff wavelengths of first dichroic mirror 12 a andsecond dichroic mirror 12 b are set to the blue-green (cyan) band thatis not used in color synthesis, whereby color synthesis can be realizedefficiently even for light that is entered at angles that differ fromparallel rays.

As with the first exemplary embodiment, the present exemplary embodimentenables the synthesis of green light from two different directions. Inaddition, a configuration is adopted that reduces the amount of redlight for which the optical output is relatively great and adds greenlight. Accordingly, the three primary colors can be synthesized atpreferable color mixing ratios and white light can be obtained havingsuperior white balance. In addition, the optical output of the LEDs ofthree colors can be displayed at their maximum without limitation.

The illumination device of the present exemplary embodiment is notlimited to the configuration that synthesizes only green light from twodirections. For example, the illumination device shown in FIG. 13 mayalso be configured such that light source 3 b further emits blue or redP-polarized light or red and blue P-polarized light.

In the illumination device of the present exemplary embodiment, fromamong red, green, and blue, the colors that undergo color mixing fromtwo directions can be set as appropriate according to design.

It is known that, due to problems in the fabrication of LEDs, the peakwavelength of an LED varies on the order of ±10-20 nm. In the firstexemplary embodiment, the cutoff wavelengths of the dichroic mirrors areset to the yellow wavelength band (at least 560 nm but no greater than600 nm), whereby the use of a color synthesis optical element in whichthe peak wavelength of green LEDs diverges in the direction of shorterwavelengths enables an even greater decrease of loss during colorsynthesis. In the second exemplary embodiment, the cutoff wavelengths ofthe dichroic mirrors are set to the blue-green (cyan) wavelength band(at least 480 nm but no greater than 500 nm), whereby the use of a colorsynthesis optical element in which the peak wavelength of green LEDsdiverges in the direction of longer wavelengths and the peak wavelengthof blue LEDs diverges in the direction of shorter wavelengths enables aneven greater decrease of loss during color synthesis. A color synthesisoptical element may thus be selected in accordance with the divergenceof the peak wavelengths of the LEDs.

In addition, the optical output characteristic of an LED varies greatlydue to problems that occur during the manufacturing process. When theoptical output of blue LEDs is relatively great, blue is decreased andgreen added as in the first exemplary embodiment. Conversely, when theoptical output of red LEDs is relatively great, red is decreased andgreen added as in the second exemplary embodiment. Still further, thecombination or arrangement of the light sources of each color can beselected by adding red or blue P-polarized light to the optical path ofgreen P-polarized light.

In this way, the illumination device of each exemplary embodiment isuseful because it allows the utilization of LEDs having great variationsin the peak wavelength or optical output.

Third Exemplary Embodiment

FIG. 19 is a perspective view showing the configuration of theillumination device that is the third exemplary embodiment of thepresent invention.

Referring to FIG. 19, the illumination device includes color synthesisoptical element 21, three light sources 23 a-23 c, and retardation plate24.

As in the first exemplary embodiment, color synthesis optical element 21is a cross dichroic prism composed of four right angle prisms 21 a-21 din which surfaces forming right angles are joined together. Firstdichroic mirror 22 a and second dichroic mirror 22 b composed ofdielectric multilayer films are formed on the joined surfaces of rightangle prisms 21 a-21 d so as to intersect.

Of the four side surfaces of color synthesis optical element 21, lightis entered from three surfaces (each of the surfaces of right angleprisms 21 a, 21 c, and 21 d) and colors are synthesized. The oneremaining side surface is the exit surface of the synthesized light.

Light source 23 a emits red light (S-polarized light). Light source 23 bemits green light (P-polarized light). Light source 23 c emits green andblue light (S-polarized light). Here, red, green, and blue correspond tothe three primary colors of light.

The S-polarized light (red) from light source 23 a is entered into colorsynthesis optical element 21 from the incident surface of right angleprism 21 c. The P-polarized light (green) from light source 23 b isentered into color synthesis optical element 21 from the incidentsurface of right angle prism 21 d. The S-polarized light (green+blue)from light source 23 c is entered into color synthesis optical element21 from the incident surface of right angle prism 21 a.

In color synthesis optical element 21, the S-polarized light (red),P-polarized light (green), and S-polarized light (green+blue) from eachof the incident surfaces are synthesized by first dichroic mirror 22 aand second dichroic mirror 22 b.

Retardation plate 24 is arranged at a position that corresponds to andfaces the exit surface (surface of right angle prism 21 c) of colorsynthesis optical element 21. The illumination device of the presentexemplary embodiment differs from illumination device of the firstexemplary embodiment in that this retardation plate 24 is provided.

In the illumination device of the first exemplary embodiment, blueS-polarized light, green P-polarized light and S-polarized light, andred S-polarized light are synthesized to obtain white light. In otherwords, luminous flux is entered in which the direction of polarizationdiffers according to light.

When the object that is illuminated has polarization dependency, theamount of light that is reflected differs according to color. In orderto circumvent this problem in the illumination device of the presentexemplary embodiment, retardation plate 24 is arranged in the directionof progression of the color-synthesized light (white light) that isexited from the exit surface of color synthesis optical element 21.

Retardation plate 24 is a quarter-wave plate, and for example, is of aconstruction in which polyvinyl alcohol film is stretched uniaxially andsandwiched between protective films. When the optical axis of aquarter-wave plate is set to the 45-degree direction, blue S-polarizedlight, green S-polarized light, and red S-polarized light becomeright-handed circularly polarized light, and green P-polarized lightbecomes left-handed circularly polarized light, whereby the directivityof the polarization of the illumination light is eliminated and thedifferences caused by color in the amount of light reflected by anobject can be resolved.

Retardation plate 24 is not limited to the above-described construction.A component composed of a multilayer film that acts as a quarter-waveplate over the broad wavelength band of white light is preferable asretardation plate 24.

Retardation plate 24 may also be a retardation plate that randomlychanges the phase difference within a minute range to cancelpolarization. The illumination light from each minute range spreads at aparticular angle, whereby polarization states that differ randomly aresuperposed to obtain illumination light that lacks a polarizationcharacteristic.

Retardation plate 24 is not limited to a film. A component that canelectronically control phase difference such as a liquid crystal elementmay also be used as retardation plate 24. A component that changesvoltage that is applied within a minute range or that changes the cellthickness of the liquid crystal to give random phase difference, or acomponent that changes phase difference at high speed to equalize thepolarization characteristic over time may also be used as retardationplate 24.

When the illuminated object has polarization dependency with respect toP-polarized light and S-polarized light, the optical axis of a half-waveplate may be set to the 22.5-degree direction and the polarized lightthat is transmitted may be rotated to the ±45-degree direction. In thiscase, reflected light is the average value of reflected light withrespect to P-polarized light and S-polarized light, and differences inreflected light arising from color can thus be canceled.

Fourth Exemplary Embodiment

FIG. 20 is a perspective view showing the configuration of theillumination device that is the fourth exemplary embodiment of thepresent invention.

Referring to FIG. 20, the illumination device includes color synthesisoptical element 21 and three light sources 23 a-23 c.

As with the first exemplary embodiment, color synthesis optical element21 is a cross dichroic prism composed of four right angle prisms 21 a-21d in which surfaces that form right angles are joined together. Firstdichroic mirror 22 a and second dichroic mirror 22 b composed ofdielectric multilayer films are formed on the joined surfaces of theright angle prisms 21 a-21 d so as to intersect.

Of the four side surfaces of color synthesis optical element 21, lightis entered from three surfaces (the surfaces of right angle prisms 21 a,21 c, and 21 d) and colors are synthesized. The one remaining sidesurface is the exit surface of light that was synthesized.

In the illumination device of the first exemplary embodiment shown inFIG. 2, light source 3 a emits red light (S-polarized light), lightsource 3 b emits green light (P-polarized light), and light source 3 cemits green and blue light (S-polarized light).

In the illumination device of the present exemplary embodiment, incontrast, light source 23 a emits red light (S-polarized light), lightsource 23 b emits blue and green light (P-polarized light), and lightsource 23 c emits green light (S-polarized light).

The S-polarized light (red) from light source 23 a is entered into colorsynthesis optical element 21 from the incident surface of right angleprism 21 c. The P-polarized light (blue and green) from light source 23b is entered into color synthesis optical element 21 from the incidentsurface of right angle prism 21 d. The S-polarized light (green) fromlight source 23 c is entered into color synthesis optical element 21from the incident surface of right angle prism 21 a. In other words, thepresent exemplary embodiment differs from the first exemplary embodimentin that green S-polarized light is entered into color synthesis opticalelement 21 from light source 23 c and blue and green P-polarized lightis entered into color synthesis optical element 21 from light source 23b.

In color synthesis optical element 21, the S-polarized light (red), theP-polarized light (blue+green), and the S-polarized light (green) fromeach incident surface are synthesized by first dichroic mirror 22 a andsecond dichroic mirror 22 b.

FIG. 21A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of firstdichroic mirror 22 a. FIG. 21B is a graph showing the spectralreflectance characteristic with respect to P-polarized light andS-polarized light of first dichroic mirror 22 a.

First dichroic mirror 22 a largely transmits and does not reflect lightof the visible range of the incident P-polarized light. The cutoffwavelengths of first dichroic mirror 22 a with respect to incidentS-polarized light are 490 nm and 580 nm. In this case, first dichroicmirror 22 a largely transmits and does not reflect S-polarized lighthaving a wavelength no greater than 490 nm and S-polarized light havinga wavelength of at least 580 nm. In addition, first dichroic mirror 22 alargely reflects and does not transmit S-polarized light of a wavelengthrange that is greater than 490 nm but less than 580 nm.

If the characteristics of first dichroic mirror 22 a are expressed asits action upon colored light, with respect to green light, firstdichroic mirror 22 a transmits P-polarized light and reflectsS-polarized light. In other words, first dichroic mirror 22 a also actsas a polarization beam splitter with respect to green light. On theother hand, with respect to blue and red light, first dichroic mirror 22a does not act in any way upon either P-polarized light or S-polarizedlight and transmits both without alteration.

FIG. 22A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of seconddichroic mirror 22 b. FIG. 22B is a graph showing the spectralreflectance characteristic with respect to P-polarized light andS-polarized light of second dichroic mirror 22 b.

The cutoff wavelength of second dichroic mirror 22 b with respect tolight entered as P-polarized light is 700 nm. In this case, seconddichroic mirror 22 b largely transmits and does not reflect P-polarizedlight having a wavelength of 700 nm or less. On the other hand, thecutoff wavelength of second dichroic mirror 22 b with respect to lightentered as S-polarized light is 580 nm. In this case, second dichroicmirror 22 b largely reflects and does not transmit S-polarized lighthaving a wavelength of at least 580 nm. Conversely, second dichroicmirror 22 b largely transmits and does not reflect S-polarized lighthaving a wavelength shorter than 580 nm.

If the characteristics of second dichroic mirror 22 b are expressed asits action upon colored light, with respect to blue and green light,second dichroic mirror 22 b doe not act in any way upon P-polarizedlight and S-polarized light and transmits both without alteration. Inaddition, with respect to red light, second dichroic mirror 22 btransmits P-polarized light and reflects S-polarized light. In otherwords, second dichroic mirror 22 b also acts as a polarization beamsplitter with respect to red light.

The setting of the cutoff wavelengths with respect to S-polarized lightof first dichroic mirror 22 a and second dichroic mirror 22 b to theblue-green (cyan) band of 490 nm and the yellow band of 580 nm differsgreatly from the spectral characteristics of the dichroic prism that wasdisclosed in Patent Document 8 (see FIGS. 1A and 1B). This point ofdifference enables the compensation of light of a color that isinsufficient within the constraints of etendue to obtain good whitebalance and allows the optical output characteristic of LED lightsources to be exhibited at a maximum. This point will be described ingreater detail hereinbelow.

FIG. 23 is a plan view for describing optical paths when colors aresynthesized using the illumination device shown in FIG. 20. Aspreviously described, of the four side surfaces of color synthesisoptical element 21, three of the surfaces are incident surfaces, andlight is entered from these incident surfaces and the colored light issynthesized by first dichroic mirror 22 a and second dichroic mirror 22b. The one remaining surface is the exit surface, and light that hasbeen synthesized by first dichroic mirror 22 a and second dichroicmirror 22 b is exited from this exit surface.

In FIG. 23, lines represented by solid lines with arrows are fordescribing representative directions of the progression of incidentluminous flux, but this does not mean that only the lines represented assolid lines with arrows are these incident light rays. The incidentlight is luminous flux having a cross-sectional area no greater than theincident surfaces of color synthesis optical element 21 and includespositions other than the lines represented by solid lines with arrows aswell as rays having an angular component.

Light source 23 c emits green S-polarized light. The green S-polarizedlight from light source 23 c is entered into color synthesis opticalelement 21 from the incident surface of right angle prism 21 a (in FIG.23, the surface located on the upper side of the figure). Seconddichroic mirror 22 b does not act in any way upon green S-polarizedlight and the green S-polarized light therefore passes through seconddichroic mirror 22 b without alteration. On the other hand, firstdichroic mirror 22 a reflects all green S-polarized light. Accordingly,the luminous flux of green S-polarized light is bent 90 degrees at firstdichroic mirror 22 a and is then exited from the exit surface of rightangle prism 21 b, as shown in FIG. 23.

Light source 23 b emits blue and green P-polarized light. The blue andgreen P-polarized light from light source 23 b is entered into colorsynthesis optical element 21 from the incident surface of right angleprism 21 d (in FIG. 23, the surface located on the left side of thefigure). Neither first dichroic mirror 22 a nor second dichroic mirror22 b act in any way upon blue and green P-polarized light, and blue andgreen P-polarized light therefore pass through each of dichroic mirrors22 a and 22 b without alteration and are then exited from the exitsurface of right angle prism 21 b.

Light source 23 a emits red S-polarized light. The red S-polarized lightfrom light source 23 a is entered into color synthesis optical element21 from the incident surface of right angle prism 21 c (in FIG. 23, thesurface located on the lower side of the figure). First dichroic mirror22 a does not act in any way upon red S-polarized light, and the redS-polarized light therefore passes through first dichroic mirror 22 awithout alteration. On the other hand, second dichroic mirror 22 breflects all red S-polarized light. As a result, luminous flux of redS-polarized light is bent 90 degrees at second dichroic mirror 22 b andis then exited from the exit surface of right angle prism 21 b, as shownin FIG. 23.

In the illumination device of the present exemplary embodiment, whitelight can thus be obtained by synthesizing blue P-polarized light, greenP-polarized light and S-polarized light, and red S-polarized light.

FIG. 24A is a graph showing the relation between the emission spectrumsof each of blue and green LED light sources and the spectraltransmittance characteristic with respect to P-polarized light of firstdichroic mirror 22 a. FIG. 24B is a graph showing the relation betweenthe emission spectrums of each of green and red LED light sources andthe spectral reflectance characteristic with respect to S-polarizedlight of first dichroic mirror 22 a. The peak wavelength of the red LEDlight source is 630 nm, the peak wavelength of the green LED lightsource is 530 nm, and the peak wavelength of the blue LED light sourceis 450 nm.

FIG. 25A is a graph showing the relation between the emission spectrumsof each of the blue and green LED light sources and the spectraltransmittance characteristic with respect to P-polarized light of seconddichroic mirror 22 b. FIG. 25B is a graph showing the relation betweenthe emission spectrums of each of the red and green LED light sourcesand the spectral reflectance characteristic with respect to S-polarizedlight of second dichroic mirror 22 b. The peak wavelength of the red LEDlight source is 630 nm, the peak wavelength of the green LED lightsource is 530 nm, and the peak wavelength of the blue LED light sourceis 450 nm.

As can be seen from FIGS. 24A and 25A, the cutoff wavelengths of firstdichroic mirror 22 a and second dichroic mirror 22 b with respect toblue and green P-polarized light are sufficiently separated.Accordingly, blue and green P-polarized light is not reflected by thesedichroic mirrors 22 a and 22 b despite shifting of the cutoffwavelengths due to incident angle dependency. As a result, loss due toincident angle dependency does not occur.

In addition, as can be seen from FIGS. 24B and 25B, the cutoffwavelengths of first dichroic mirror 22 a and second dichroic mirror 22b with respect to red S-polarized light and green S-polarized light aresufficiently separated. Accordingly, red and green S-polarized light canbe synthesized by these dichroic mirrors 22 a and 22 b with virtually noloss despite shifting of the cutoff wavelengths due to incident angledependency.

The cutoff wavelengths of first dichroic mirror 22 a and second dichroicmirror 22 b are thus set to the blue-green (cyan) and yellow wavelengthbands that are not used in color synthesis, whereby colored light can beefficiently synthesized for light that is entered at angles that differfrom parallel light.

As in the first exemplary embodiment, the present exemplary embodimentenables the synthesis of green light from two different directions.Moreover, a configuration is adopted that decreases the amount of bluelight for which optical output is relatively great and adds green light.Accordingly, the three primary colors can be synthesized at preferablecolor mixture ratios, and white light having superior white balance canbe obtained. In addition, the optical output of the LEDs of three colorscan be exhibited at the maximum without limitation.

The illumination device of the present exemplary embodiment is notlimited to a configuration that synthesizes only green light from twodifferent directions. For example, in the illumination device shown inFIG. 20, light source 23 b may be of a configuration that further emitsred P-polarized light.

Fifth Exemplary Embodiment

FIG. 26 is a perspective view showing the configuration of theillumination device that is the fifth exemplary embodiment of thepresent invention.

Referring to FIG. 26, the illumination device includes color synthesisoptical element 31 and three light sources 33 a-33 c.

As with the first exemplary embodiment, color synthesis optical element31 is a cross dichroic prism composed of four right angle prisms 31 a-31d in which surfaces that form right angles are joined together. Firstdichroic mirror 32 a and second dichroic mirror 32 b that are composedof dielectric multilayer films are formed on the joined surfaces ofright angle prisms 31 a-31 d so as to intersect.

Of the four side surfaces of color synthesis optical element 31, lightis entered from three surfaces (each of the surfaces of right angleprisms 31 a, 31 c, and 31 d). The one remaining side surface is the exitsurface of light that has been synthesized.

In the illumination device of the first exemplary embodiment shown inFIG. 2, light source 3 a emits red light (S-polarized light), lightsource 3 b emits green light (P-polarized light), and light source 3 cemits green and blue light (S-polarized light). In the illuminationdevice of the present exemplary embodiment, in contrast, light source 33a emits blue light (S-polarized light), light source 33 b emits red andgreen light (P-polarized light), and light source 33 c emits green light(S-polarized light).

The S-polarized light (blue) from light source 33 a is entered intocolor synthesis optical element 31 from the incident surface of rightangle prism 31 c. The P-polarized light (red+green) from light source 33b is entered into color synthesis optical element 3 lfrom the incidentsurface of right angle prism 31 d. The S-polarized light (green) isentered into color synthesis optical element 31 from the incidentsurface of right angle prism 31 a. In other words, the present exemplaryembodiment differs from the first exemplary embodiment in that greenS-polarized light is entered into color synthesis optical element 31from light source 33 c, red and green P-polarized light is entered intocolor synthesis optical element 31 from light source 33 b, and blueS-polarized light is entered into color synthesis optical element 31from light source 33 a.

In color synthesis optical element 31, the S-polarized light (blue),P-polarized light (red+green), and S-polarized light (green) from eachincident surface are synthesized by first dichroic mirror 32 a andsecond dichroic mirror 32 b.

FIG. 27A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of firstdichroic mirror 32 a. FIG. 27B is a graph showing the spectralreflectance characteristic with respect to P-polarized light andS-polarized light of first dichroic mirror 32 a.

First dichroic mirror 32 a largely transmits and does not reflect lightof the visible range of the incident P-polarized light. The cutoffwavelengths of first dichroic mirror 32 a with respect to incidentS-polarized light are 490 nm and 580 nm. In this case, first dichroicmirror 32 a largely transmits and does not reflect S-polarized lighthaving a wavelength of 490 nm or less and 580 nm or more. In addition,first dichroic mirror 32 a largely reflects and does not transmitS-polarized light having a wavelength band that is greater than 490 nmbut less than 580 nm.

If the characteristics of first dichroic mirror 32 a are expressed asits action upon colored light, with respect to green light, firstdichroic mirror 32 a transmits P-polarized light and reflectsS-polarized light. In other words, first dichroic mirror 32 a also actsas a polarization beam splitter with respect to green light. Firstdichroic mirror 32 a does not act in any way upon blue and red light andtransmits both P-polarized light and S-polarized light.

FIG. 28A is a graph showing the spectral transmittance characteristicwith respect to P-polarized light and S-polarized light of seconddichroic mirror 32 b. FIG. 28B is a graph showing the spectralreflectance characteristic with respect to P-polarized light andS-polarized light of second dichroic mirror 32 b.

The cutoff wavelength with respect to incident P-polarized light ofsecond dichroic mirror 32 b is 400 nm. In this case, second dichroicmirror 32 b largely transmits and does not reflect P-polarized lighthaving a wavelength of 400 nm or more. In addition, the cutoffwavelength of second dichroic mirror 32 b with respect to incidentS-polarized light is 490 nm. In this case, second dichroic mirror 32 blargely reflects and does not transmit S-polarized light having awavelength no greater than 490 nm. In addition, second dichroic mirror32 b largely transmits and does not reflect S-polarized light having awavelength longer than 490 nm.

If the characteristics of second dichroic mirror 32 b are expressed byits action upon colored light, with respect to red and green light,second dichroic mirror 32 b does not act in any way upon P-polarizedlight and S-polarized light and transmits both without alteration. Withrespect to blue light, second dichroic mirror 32 b transmits P-polarizedlight and reflects S-polarized light. In other words, second dichroicmirror 32 b also acts as a polarization beam splitter upon blue light.

The present exemplary embodiment differs greatly from the dichroic prismdisclosed in Patent Document 8 (see FIG. 1A and FIG. 1B) in that thecutoff wavelengths with respect to S-polarized light of first dichroicmirror 32 a and second dichroic mirror 32 b are set to the blue-green(cyan) band of 490 nm and the yellow band of 580 nm. Due to this pointof difference, light of a color that is insufficient can be augmentedwithin the constraints of etendue to obtain good white balance and theoptical output characteristic of LED light sources can be exhibited totheir maximum. These features are explained in greater detailhereinbelow.

FIG. 29 is a plan view for describing the optical paths when colors aresynthesized using the illumination device shown in FIG. 26. Aspreviously described, three surfaces of the four side surfaces of colorsynthesis optical element 31 are incident surfaces, and light is enteredfrom these incident surfaces, and the colored light is synthesized byfirst dichroic mirror 32 a and second dichroic mirror 32 b. The oneremaining surface is the exit surface and the light that is synthesizedby first dichroic mirror 32 a and second dichroic mirror 32 b is exitedfrom this exit surface.

In FIG. 29, lines represented as solid lines with arrows are fordescribing the representative directions of the progression of incidentluminous flux, but this does not mean that only the lines represented assolid lines with arrows are the incident rays. The incident light isluminous flux having a cross-sectional area no greater than the incidentsurfaces of color synthesis optical element 31 and includes positionsother than the lines represented as solid lines with arrows as well asrays having an angular component.

Light source 33 c emits green S-polarized light. Green S-polarized lightfrom light source 33 c is entered into color synthesis optical element31 from the incident surface of right angle prism 31 a (in FIG. 29, thesurface located on the upper side of the figure). Second dichroic mirror32 b does not act in any way upon green S-polarized light, and greenS-polarized light therefore passes through second dichroic mirror 32 bwithout alteration. On the other hand, first dichroic mirror 32 areflects all green S-polarized light. As a result, the luminous flux ofgreen S-polarized light is bent 90 degrees by first dichroic mirror 32 aand is then exited from the exit surface of right angle prism 31 b, asshown in FIG. 29.

Light source 33 b emits red and green P-polarized light. The red andgreen P-polarized light from light source 33 b is entered into colorsynthesis optical element 31 from the incident surface of right angleprism 31 d (in FIG. 29, the surface located on the left side of thefigure). Neither first dichroic mirror 32 a nor second dichroic mirror32 b act in any way upon red and green P-polarized light, and the redand green P-polarized light therefore pass through each of dichroicmirrors 32 a and 32 b without alteration and are then exited from theexit surface of right angle prism 31 b.

Light source 33 a emits blue S-polarized light. The blue S-polarizedlight from light source 33 a is entered into color synthesis opticalelement 31 from the incident surface of right angle prism 31 c (in FIG.29, the surface located on the lower side of the figure). First dichroicmirror 32 a does not act in any way upon blue S-polarized light, and theblue S-polarized light therefore passes through first dichroic mirror 32a without alteration. On the other hand, second dichroic mirror 32 breflects all blue S-polarized light. As a result, luminous flux of blueS-polarized light is bent 90 degrees at second dichroic mirror 32 b andthen exited from the exit surface of right angle prism 31 b, as shown inFIG. 29.

Thus, in the illumination device of the present exemplary embodiment,white light can be obtained by synthesizing blue S-polarized light,green P-polarized light and S-polarized light, and red P-polarizedlight.

FIG. 30A is a graph showing the relation between the emission spectrumsof each of green and red LED light sources and the spectraltransmittance characteristic with respect to P-polarized light of firstdichroic mirror 32 a. FIG. 30B is a graph showing the relation betweenthe emission spectrums of each of blue and green LED light sources andthe spectral reflectance characteristic with respect to S-polarizedlight of first dichroic mirror 32 a. The peak wavelength of the red LEDlight source is 630 nm, the peak wavelength of the green LED lightsource is 530 nm, and the peak wavelength of the blue LED light sourceis 450 nm.

FIG. 31A is a graph showing the relation between the emission spectrumsof each of red and green LED light sources and the spectraltransmittance characteristic with respect to P-polarized light of seconddichroic mirror 32 b. FIG. 31B is a graph showing the relation betweenthe emission spectrums of each of the blue and green LED light sourcesand the spectral reflectance characteristic with respect to S-polarizedlight of second dichroic mirror 32 b. The peak wavelength of the red LEDlight source is 630 nm, the peak wavelength of the green LED lightsource is 530 nm, and the peak wavelength of the blue LED light sourceis 450 nm.

As can be clearly seen from FIGS. 30A and 31A, the cutoff wavelengthswith respect to red and green P-polarized light of first dichroic mirror32 a and second dichroic mirror 32 b are sufficiently separated.Accordingly, blue and green P-polarized light is not reflected by thesedichroic mirrors 32 a and 32 b despite shifting of the cutoffwavelengths due to incident angle dependency. As a result, loss due toincident angle dependency does not occur.

In addition, as can be clearly seen from FIGS. 30B and 31B, the cutoffwavelengths with respect to blue S-polarized light and green S-polarizedlight of first dichroic mirror 32 a and second dichroic mirror 32 b aresufficiently separated. Accordingly, blue and green S-polarized lightcan be synthesized by these dichroic mirrors 32 a and 32 b withvirtually no loss despite shifting of the cutoff wavelengths due toincident angle dependency.

Because the cutoff wavelengths of first dichroic mirror 32 a and seconddichroic mirror 32 b are thus set to the blue-green (cyan) and yellowwavelength bands that are not used in color synthesis, colored light canbe efficiently synthesized even for light that is entered at angles thatdiffer from parallel light.

According to the present exemplary embodiment, as in the first exemplaryembodiment, green light can be synthesized from two differentdirections. In addition, a configuration is adopted that decreases theamount of red light for which optical output is relatively great andthat adds green light. Accordingly, the three primary colors can besynthesized at preferable color mixture ratios, and white light isobtained that has superior white balance. In addition, the opticaloutput of the LEDs of three colors can be exhibited at the maximumwithout limitations.

The illumination device of the present exemplary embodiment is notlimited to a configuration in which only green light is synthesized fromtwo different directions. For example, in the illumination device shownin FIG. 26, light source 33 b may be of a configuration that furtheremits blue P-polarized light.

Sixth Exemplary Embodiment

FIG. 32 is a block diagram showing the configuration of theprojection-type display device that is the sixth exemplary embodiment ofthe present invention.

Referring to FIG. 32, the projection-type display device includesillumination device 100, control means 110, display element 120, andprojection lens 130.

Illumination device 100 is of the same configuration as the illuminationdevice of the first exemplary embodiment and is made up of colorsynthesis optical element 1 and three light sources 3 a-3 c. Colorsynthesis optical element 1 is a cross dichroic prism in which firstdichroic mirror 2 a and second dichroic mirror 2 b composed ofdielectric multilayer films are formed on the joined surfaces of fourright angle prisms so as to intersect.

Light is entered from three surfaces (the surfaces of each of rightangle prisms 1 a, 1 c, and 1 d) of the four side surfaces of colorsynthesis optical element 1, and colors are synthesized. The oneremaining side surface is the exit surface of light that has beensynthesized. An anti-reflection film composed of a dielectric multilayerfilm is applied to the incident surfaces and exit surface of colorsynthesis optical element 1.

Light source 3 a emits red light (S-polarized light). Light source 3 bemits green light (P-polarized light). Light source 3 c emits green andblue light (S-polarized light). Here, red, green, and blue correspond tothe three primary colors of light.

The S-polarized light (red) from light source 3 a is entered into colorsynthesis optical element 1 from the incident surface of right angleprism 1 c. The P-polarized light (green) from light source 3 b isentered into color synthesis optical element 1 from the incident surfaceof right angle prism 1 d. The S-polarized light (blue) from light source3 c is entered into color synthesis optical element 1 from the incidentsurface of right angle prism 1 a.

In color synthesis optical element 1, the S-polarized light (red),P-polarized light (green), and S-polarized light (green+blue) from eachof the incident surfaces is synthesized by first dichroic mirror 2 a andsecond dichroic mirror 2 b, and the synthesized light (white light) isexited from the exit surface.

Display element 120 is arranged in the direction of progression of thelight that is exited from the exit surface of color synthesis opticalelement 1, and projection lens 130 is arranged in the direction ofprogression of light that is reflected by display element 120.Projection lens 130 projects the image displayed on display element 120onto a screen (not shown).

As shown in FIG. 5, each of light sources 3 a-3 c is made up of LEDmodule 50, condensing optics 52, and drive circuit 53. In LED module 50,an LED that is light-emitting unit 51 is mounted on a substrate. Thesubstrate additionally serves as a heat radiation plate and a heat sink(not shown) is attached. In addition, heat control is effected by aforced cooling apparatus to stabilize the light emission characteristicof the LED.

Control means 110 controls light sources 3 a-3 c and display element120. FIG. 33 shows the configuration of this control means 110. In FIG.33, for the sake of convenience, illumination device 100 and displayelement 120 are shown in addition to control means 110. Illuminationdevice 100 shown in FIG. 33 is composed of light source 3 and colorsynthesis optical element 1. Light source 3 corresponds to light sources3 a-3 c shown in FIG. 32, but in FIG. 33, the light sources are shown ina simplified state as a single light source.

As shown in FIG. 33, control means 110 is provided with video signalprocessing circuit 111. Video signal processing circuit 111 bothsuccessively supplies image signals of each color component from theinput video signal to drive circuit 122 of display element 120 to bringabout the display of images for each color component on display element120 and supplies synchronizing signals that are matched to the timing ofthe image display of each color component to drive circuit 53 of eachlight source of illumination device 100 to control lighting of the lightsource of the color that corresponds to the color component for theimage display that is being effected.

Drive circuit 53 that has received a synchronizing signal from videosignal processing circuit 111 supplies current to the LED that is thelight-emitting unit of LED module 50. The light from the LED iscondensed by condensing optics 52, and the condensed light is thenentered into the incident surfaces of color synthesis optical element 1.

As condensing optics 52, a lens-shaped optical element is used in FIG.33, but a reflective optical element such as a reflector may also beused. In addition, a fly-eye lens or glass rod may also be used as anintegrator for causing the light to illuminate display element 120uniformly. Polarization conversion optics that employ a polarizationbeam splitter and half-wave plate may also be used to efficiently obtainthe light of the polarization component that is used in color synthesisoptical element 1. Of course, the light-emitting unit of LED module 50may be the light source that produces polarized light, or a polarizationconversion function may be provided in the light-emitting unit togenerate polarized light from the light-emitting unit. In either form,the light source can be constructed by means of any combination of knowntechnology.

Referring again to FIG. 32, light source 3 a is provided with red LEDmodule 60 shown in FIG. 6A, light source 3 b is provided with green LEDmodule 70 shown in FIG. 6B, and light source 3 c is provided with blueLED module 80 shown in FIG. 6C.

The light-emitting unit of red LED module 60 is composed of four LEDchips 61 a-61 d for which the peak wavelength is 630 nm. Thelight-emitting unit of green LED module 70 is composed of four LED chips71 a-71 d for which the peak wavelength is 520 nm. The light-emittingunit of blue LED module 80 is composed of three LED chips 81 a-81 c forwhich the peak wavelength is 460 nm and one LED chip 81 d for which thepeak wavelength is 520 nm. The emission spectrums of the red, green, andblue LED chips are the same as those shown in FIG. 8B.

The areas of the light-emitting units of each of red LED module 60,green LED module 70, and blue LED module 80 are determined by the areaof the display element and the f-number of the projection lens based onthe constraints of etendue, but in the determination of the areas of thelight-emitting units, factors such as the positioning margins infabrication and the uniformity of the illuminance distribution of theillumination light are taken into consideration.

In the red LED module 60, green LED module 70, and blue LED module 80,light-emission characteristics vary with respect to current of the LEDchips that make up the light-emitting units, and drive circuit 53 shownin FIG. 33 therefore controls the amount of current to the LED chipsaccording to these light emission characteristics.

The characteristics of the LEDs of each color at the time of rated driveare described next. The chromaticity of a red LED is given as (0.700,0.300) on xy chromaticity coordinates, and the emitted luminous flux is455 lm per chip. The chromaticity of the green LED is given as (0.195,0.700) on xy chromaticity coordinates, and the emitted luminous flux is1000 lm per chip. The chromaticity of the blue LED is given as (0.140,0.046) on xy chromaticity coordinates, and the emitted luminous flux is133 lm per chip.

Display element 120 is, for example, an MEMS (Micro Electro MechanicalSystem)-type element in which a micromirror is provided for each pixeland that switches light by the action of mirrors. According to thiselement, a color picture can be displayed by a field-sequential mode.

Video signal processing circuit 111 breaks down one frame of a pictureinto fields of each of the colors red, green, and blue by time andcauses display of the images of the color components of each field ondisplay unit 121 by way of drive circuit 122. Video signal processingcircuit 111 supplies synchronizing signals to drive circuit 53 ofillumination device 100 in synchronization with the field image displayand drives the light source of each color to switch the colors of theillumination light. Half-tones can be reproduced by modulating the pulsewidth of the signals that are supplied to drive circuit 53.

The colors of field images that are displayed are not limited to thethree colors red, green, and blue. Field images of C (cyan), M(magenta), Y (yellow) and W (white) in which these colors are combinedmay also be displayed. In this case, a plurality of light sources amonglight sources 3 a-3 c are lit simultaneously in accordance with thecolor of the field image that is to be displayed.

Display element 120 is not limited to a device that uses micromirrors.Instead of a reflective type, display element 120 may also be atransmissive-type display element in which a microshutter is provided ineach pixel. Of course, display element 120 may also be a display elementother than the MEMS type. However, two polarization components areincluded in illumination light, and a device that has no polarizationdependence in display operations is therefore preferable as displayelement 120. When there is polarization dependency in the displayoperations of display element 120, polarization conversion optics may beused to arrange to one polarization component.

The spectral transmittance characteristic and spectral reflectancecharacteristic with respect to P-polarized light and S-polarized lightof first dichroic mirror 2 a are as shown in FIGS. 3A and 3B.

The spectral transmittance characteristic and spectral reflectancecharacteristic with respect to P-polarized light and S-polarized lightof second dichroic mirror 2 b are as shown in FIGS. 4A and 4B.

The operations of the projection-type display device composed of theabove-described constituent elements are next described using FIG. 32.

Light emitted from light source 3 a (light emitted from red LED module60) has the spectrum of red LEDs and is entered into color synthesisoptical element 1 as S-polarized light. S-polarized light having thespectrum of red LEDs passes through first dichroic mirror 2 a (see FIG.8B) and is reflected by second dichroic mirror 2 b (see FIG. 9B). Inother words, the optical path of red S-polarized light is bent 90degrees at second dichroic mirror 2 b and the red S-polarized light isthen exited from the exit surface of color synthesis optical element 1.

Light emitted from light source 3 b (light emitted from green LED module70) has the spectrum of green LEDs and is entered into color synthesisoptical element 1 as P-polarized light. The P-polarized light having thespectrum of green LEDs passes through first dichroic mirror 2 a (seeFIG. 8B) and also passes through second dichroic mirror 2 b (see FIG.9B). In other words, P-polarized light having the spectrum of green LEDsis exited from the exit surface of color synthesis optical element 1unchanged without having its optical path bent.

Light emitted from light source 3 c (light emitted from blue LED module80) has the spectrum of a green LED in addition to the spectrum of blueLEDs and is entered into color synthesis optical element 1 asS-polarized light. The S-polarized light having the spectrum of blueLEDs and the spectrum of a green LED passes through second dichroicmirror 2 b (see FIG. 9B) and is reflected by first dichroic mirror 2 a(see FIG. 8B). In other words, S-polarized light having the spectrum ofblue LEDs and the spectrum of a green LED has its optical path bend 90degrees at first dichroic mirror 2 a and then is exited from the exitsurface of color synthesis optical element 1.

Illumination light (red) that is entered into display element 120 fromlight source 3 a by way of color synthesis optical element 1 has anangular spread in the order of ±15 degrees. Similarly, illuminationlight (green) entered into display element 120 from light source 3 b byway of color synthesis optical element 1 and illumination light(green+blue) entered into display element 120 from light source 3 c byway of color synthesis optical element 1 both have angular spread in theorder of ±15 degrees. According to the present exemplary embodiment, thecutoff wavelengths with respect to green P-polarized light of firstdichroic mirror 2 a and second dichroic mirror 2 b are sufficientlyseparated. Accordingly, green P-polarized light is not reflected bythese dichroic mirrors 2 a and 2 b despite shifting of the cutoffwavelengths due to incident angle dependency. As a result, loss does notoccur due to incident angle dependency.

Similarly, the cutoff wavelengths with respect to blue and greenS-polarized light and red S-polarized light of first dichroic mirror 2 aand second dichroic mirror 2 b are sufficiently separated. Accordingly,blue, green, and red S-polarized light can be synthesized with virtuallyno loss by these dichroic mirrors 2 a and 2 b despite shifting of thecutoff wavelengths due to incident angle dependency.

As described hereinabove, light from light sources 3 a-3 c is enteredinto color synthesis optical element 1 from three directions. In colorsynthesis optical element 1, light entered from three directions issynthesized and exited so as to have the same optical axis, and displayelement 120 is illuminated by the light exited from this color synthesisoptical element 1. The luminous flux that has undergone intensitymodulation according to the image in display element 120 is entered intoprojection lens 130. The images (pictures) displayed in display element120 are then projected on a screen (not shown) by projection lens 130.

The effect of the projection-type display device of the presentexemplary embodiment is next described.

As an example, the light-emitting unit of a blue LED module is made upof four blue LEDs, the light-emitting unit of a green LED module is madeup of four green LEDs, and the light-emitting unit of a red LED moduleis made up of four red LEDs. The total synthesized luminous flux whenthe luminous flux from each of these blue, green, and red LED modules issynthesized is 6352 lm (455+1000+133).

However, the chromaticity of white that is synthesized as describedabove is (0.299, 0.271) and therefore diverges greatly in the directionof blue-violet from the white chromaticity (0.313, 0.329) of thestandard illuminant D65. This divergence occurs because the opticaloutput of green LEDs is relatively weak with respect to the lightquantity ratios for obtaining desirable white while the optical outputof blue LEDs is relatively strong.

To obtain white balance, the emitted luminous flux of green must beincreased. If within the rated range, the emitted luminous flux can beincreased by increasing the current that flows to the LEDs. However,increasing the current amount in a state in which the emitted luminousflux from green LEDs is 1000 lm results in driving the LED at a levelthat exceeds its rating, and in this case, the luminous flux cannot beexpected to increase in accordance with an increase of the currentamount. In addition, driving LEDs in excess of the rating not onlyshortens the life expectancy of the LEDs but may even destroy the LEDs.

Based on the foregoing, the emitted luminous flux of the blue LEDs isnormally suppressed to from 133 lm to 80 lm and the emitted luminousflux of the red LEDs is suppressed to from 455 lm to 364 lm inaccordance with the emitted luminous flux of the green LEDs. In thiscase, the total luminous flux is 5776 lm, and the brightness isdecreased by 9%.

In the projection-type display device of the present exemplaryembodiment, in contrast, blue LED module 80 is made up of three LEDchips 81 a-81 c that emit blue light and a single LED chip 81 d thatemits green light, as shown in FIG. 6C. In other words, compared to theabove-described blue LED module that is made up of four blue LEDs, thenumber of blue LED chips in this blue LED module is reduced by one tothree and an LED chip that emits green light is arranged in its place.

In addition, in the projection-type display device of the presentexemplary embodiment, red LED module 60 is made up of four LED chips 61a-61 d that emit red light as shown in FIG. 6A, and green LED module 70is made up of four LED chips 71 a-71 d that emit green light as shown inFIG. 6B. Accordingly, the number of green LED chips is the four LEDchips 71 a-71 d that are provided in green LED module 70 and the singlegreen LED chip 81 d that is used in blue LED module 80 for a total offive. In addition, the number of blue LED chips is three, and the numberof red LED chips is four. When these red, green, and blue LED chips areall driven at rating, the white chromaticity (0.313, 0.329) of standardilluminant D65 is obtained as the white chromaticity. In addition, thetotal luminous flux is 7219 lm, enabling a 25% improvement over the 5776lm described above.

According to the present exemplary embodiment as described hereinabove,a projection-type display device is obtained that can display a brightprojected image by using an illumination device that can exhibit theoptical output performance of LEDs at a maximum, that obtains whitelight having superior white balance, and moreover, that features highoptical utilization efficiency when mixing colors.

In the present exemplary embodiment, a cross dichroic prism shown in thefirst exemplary embodiment was used as color synthesis optical element1, but when the optical output characteristic of red LEDs surpasses thatof blue LEDs, the cross dichroic prism shown in the second exemplaryembodiment may also be used. In this case, four LED chips that emitgreen light are mounted in the green LED module, four LED chips thatemit blue light are mounted in the blue LED module, and three LED chipsthat emit red light and one LED chip that emits green light are mountedin the red LED module.

As other examples, the number of green LEDs may be decreased and red orblue LEDs may be added in the green LED module.

Blue LED module 80 shown in FIG. 6C was assumed to have three blue LEDchips 81 a-81 c and one green LED chip 81 d mounted on a substrate, butthe module is not limited to this configuration. Four green LED ofone-quarter the chip area may be used and these may be arranged insymmetrical form, for example, in the four corners of the light-emittingunit, whereby the color mixing of the emitted light is improved.

All of the LED modules of each color shown in FIGS. 6A-6C have four LEDchips mounted on a substrate, but the modules are not limited to thisform. The LED chip that is mounted in an LED module that emits light ofa single color may be a single LED chip having four times the area. Thenumber of LED chips that are mounted in an LED module that emits lightof two colors may be two or more. The important point is not the numberof LED chips, but rather, the chip area. The chip area of LED chips thatare mounted in an LED module is preferably set while keeping thecolor-mixing ratio in mind. Using LED chips having a small area enablessetting the chip area at a more precise color-mixing ratio.

Of course, if the optical output characteristic of blue LEDs is higher,two blue LED chips in the blue LED module shown in FIG. 6C may bereplaced by two green LED chips. An LED module is therefore preferablydesigned as appropriate according to the optical output characteristicsof the LEDs that are used.

Alternatively, rather than mounting a plurality of LED chips on a singlesubstrate, a plurality of components each having one LED chip mountedmay be used and synthesis realized using an optical means such as alight guide plate.

Still further, to increase the absolute quantity of light, means may beused in combination that use a hologram or dichroic mirror described inthe background art to synthesize light of a plurality of colors havingdifferent peak wavelengths.

In the interest of simplification in the foregoing explanation, thedisplay elements of each color, the color synthesis optical element, andthe projection lens were assumed to be components that do not generateloss that depends on wavelength, and explanation was presented usingratios of the quantity of luminous flux emitted from each light source.In actuality, there are also constituent parts in which the transmissioncharacteristics vary according to color, and the area ratios of LEDchips are preferably set by the light-quantity ratios of luminous fluxof each color exited from the projection lens when displaying anall-white screen.

Seventh Exemplary Embodiment

FIG. 34 is a block diagram showing the configuration of theprojection-type display device that is the seventh exemplary embodimentof the present invention. The projection-type display device of thepresent exemplary embodiment is a device in which retardation plate 140is added to the configuration of the projection-type display device ofthe sixth exemplary embodiment. The configuration and operations otherthan retardation plate 140 are all identical to the sixth exemplaryembodiment.

As described hereinabove, in the projection-type display device of thesixth exemplary embodiment, blue S-polarized light, green P-polarizedlight and S-polarized light, and red S-polarized light are synthesizedin color synthesis optical element 1, and the light from this colorsynthesis optical element 1 illuminates display element 120. When, forexample, an MEMS-type component is used for display element 120, thepolarization direction also differs for each color for a projectedimage.

A case is considered in which the projection-type display device of thesixth exemplary embodiment is applied to a stereoscopic display devicethat uses a liquid crystal shutter eyeglasses to perform stereoscopicdisplay. In a stereoscopic display device of this type, a screen is usedthat allows the polarization of the projected image to be maintained. Asa result, when the image that is projected onto the screen from theprojection-type display device is viewed by way of liquid crystalshutter eyeglasses, the polarization direction of transmitted light islimited by a sheet polarizer that is provided in the liquid crystalshutter eyeglasses. In other words, the colors of the image are changed.A similar problem occurs not only with liquid crystal shuttereyeglasses, but whenever a means that limits polarization is used toview a projected image.

In the projection-type display device of the present exemplaryembodiment, retardation plate 140 is provided between color synthesisoptical element 1 and display element 120 at a position that faces theexit surface of color synthesis optical element 1 to solve theabove-described problem.

Retardation plate 140 is a quarter-wave plate and is of a configurationin which a polyvinyl alcohol film is stretched uniaxially and sandwichedbetween protective films. When, for example, the optical axis ofretardation plate 140 is set to the 45-degree direction, blueS-polarized light, green S-polarized light, and red S-polarized lightbecome right-handed circularly polarized light and green P-polarizedlight becomes left-handed circularly polarized light, whereby thedirectivity of the polarization of projected light can be eliminated andthe problem of differences in the amount of reflected light due to colorcan be canceled.

Retardation plate 140 is not limited to the construction describedabove. As retardation plate 140, a preferable construction is made up ofa multilayer film that acts as a quarter-wave plate across the broadwavelength band of white light.

In addition, retardation plate 140 may also be a retardation plate thatmakes phase difference change randomly in minute regions to thus cancelpolarization. Illumination light from each minute region progressivelyspreads at a certain angle, whereby randomly different polarizationstates are superposed and illumination light lacking polarization isobtained.

Retardation plate 140 is not limited to a film. A component that canelectronically control phase difference such as a liquid crystal elementcan also be used as retardation plate 140. In addition, a component thatchanges the voltage that is applied in minute regions to thus change theliquid crystal cell thickness and give random phase differences may alsobe used as retardation plate 140.

When the polarization axis of the sheet polarizer of liquid crystalshutter eyeglasses is in a direction parallel to P-polarized light orS-polarized light, a half-wave plate may be used as retardation plate140, and the optical axis may be set to the 22.5-degree direction torotate the polarized light that passes through in the ±45-degreedirection. In this case, light that passes through liquid crystalshutter eyeglasses is the average value of transmitted light withrespect to P-polarized light and S-polarized light and the problem ofdifferences in transmitted light due to color can therefore be resolved.

The position at which retardation plate 140 is arranged is not limitedto a position that faces the exit surface of color synthesis opticalelement 1. Retardation plate 140 may also be arranged between displayelement 120 and projection lens 130 or at a position that faces the exitsurface of projection lens 130. In either case, the same effect can beobtained as when retardation plate 140 is arranged at a position thatfaces the exit surface of color synthesis optical element 1.

The present invention realized by each of the above-described exemplaryembodiments exhibits the following actions and effects.

Typically, when solid-state light sources such as LEDs are used as eachof red, green, and blue light sources and the red, green, and blue lightfrom each solid-state light source is synthesized to obtain white lighthaving superior white balance, the blue optical output with respect tothe color mixing ratios of red, green, and blue light is greater thanfor the other colors and the green optical output is smaller than forthe other colors. In such cases, the optical output of the blue and redsolid-state light sources is limited to accord with the greensolid-state light source for which the optical output is relativelysmall, whereby the optical output of the white light that is obtained isreduced.

The illumination device according to one aspect of the present inventionincludes:

a first light source that includes a solid-state light source whose peakwavelength is set in the red wavelength band;

a second light source that includes a solid-state light source whosepeak wavelength is set in the green wavelength band;

a third light source that includes a solid-state light source whose peakwavelength is set in the blue wavelength band; and

a color synthesis optical element in which colored light of a firstpolarization that is entered from the second light source and coloredlight of a second polarization for which the state of polarizationdiffers from the first polarization that is entered from the first andthird light sources are synthesized;

wherein any one of the first to third light sources further includes atleast one solid-state light source whose peak wavelength is set in aspecific wavelength band that is the wavelength band of the color of thesolid-state light source that is used in one of the remaining two lightsources.

According to the above-described configuration, green light can besynthesized from two different directions. In addition, thisconfiguration decreases the amount of blue light for which opticaloutput is relatively great and adds green light. Accordingly, the threeprimary colors can be synthesized at optimum color mixing ratios andwhite light is obtained having superior white balance.

In addition, the optical output of the solid-state light sources of thethree colors can be exhibited at a maximum without limitations.

In the above-described illumination device of the present invention, theabove-described color synthesis optical element may be of aconfiguration that includes: an exit surface;

first to third incident surfaces;

first and second films that are provided such that the film surfacesintersect each other and that selectively reflect or transmit incidentlight according to wavelength;

the above-described first film transmitting, of visible light of a firstpolarization, at least light of the above-described specific wavelengthband and reflecting, of visible light of a second polarization whosepolarization state differs from that of the above-described firstpolarization, at least light of the above-described specific wavelengthband;

the above-described second film transmitting, of visible light of theabove-described first polarization, at least light of theabove-described specific wavelength band and transmitting, of visiblelight of the above-described second polarization, at least light of theabove-described specific wavelength band; and

the cutoff wavelengths with respect to the above-described secondpolarization of the above-described first and second films being setwithin band ranges other than the wavelength bands of red, green, andblue that are the three primary colors of light, and at least coloredlight of the above-described second polarization that is entered fromthe above-described first incident surface, colored light of theabove-described first polarization that is entered from theabove-described second incident surface, and colored light of theabove-described second polarization that is entered from theabove-described third incident surface being exited from theabove-described exit surface by way of the above-described first andsecond films.

The following actions and effects are exhibited by means of thisconfiguration. In the following explanation, the problems in thepreviously described Patent Documents 1-5 are also described together.

Typically, a dichroic mirror composed of a dielectric multilayer film,while having the advantage of small light absorption, also has incidentangle dependency and polarization dependency. Although incident angledependency and polarization dependency do not occur in the case ofperpendicular incidence (the angle of incidence is 0 degrees), thisdichroic mirror has the characteristic in which the amount of shift ordivergence from the designed value of cutoff wavelength increases as theangle of incidence increases.

In addition, the cut-off characteristic is not steep, but rather, has aslope in a band in the order of 20 nm-30 nm, whereby theseparation-synthesis efficiency in this wavelength band drops.

As shown in Patent Document 1, regarding light from LEDs for which peakwavelengths are close, the synthesis efficiency drops in the vicinity ofthe cutoff wavelengths. When an LED having a large semiconductor chiparea is used to obtain brightness, despite conversion to parallelluminous flux by means of a lens, light exited from directions otherthan the optical axis and having angularity is exited from the lens,whereby the colors of the synthesized light will differ for each angularcomponent due to the incident angle dependency of the dichroic mirrors.In addition, the emitted light from the LED has random polarizationdirections, whereby only the component of any one of the polarizationdirections may be synthesized.

When this type of dichroic mirror is used to synthesize light of aplurality of colors, the efficiency during synthesis drops when the peakwavelengths are not separated and a bright synthesized light cannot beobtained.

As shown in Patent Documents 2-5, when four colors or six colors amongthe band of white light are synthesized, when bright luminous flux is tobe obtained, light other than parallel luminous flux also occurs and theefficiency of light synthesis similarly drops due to incident angledependency or polarization dependency. Furthermore, when the ratios atwhich multiple colors are synthesized differ due to angular components,irregular color will appear on the projection screen.

In particular, as shown in Patent Document 2 or Patent Documents 4 and5, when light from LEDs of two colors is supplied from the samedirection, the colored light does not mix as parallel luminous flux, andangular spread must be conferred in order to achieve uniform colormixing of each of the colors. However, when angular spread is conferred,loss occurs during color mixing of light that is entered from otherdirections due to the incident angle dependency of the dichroic mirrors.Thus, there is the trade-off in which although angular spread must beconferred in order to achieve uniform color mixing of each of thecolors, conferring this angular spread results in loss due to incidentangle dependency.

According to the color synthesis optical element in the illuminationdevice of the above-described present invention, a configuration can beprovided in which the cutoff wavelengths of a first film (for example, adichroic mirror) and a second film (for example, a dichroic mirror) withrespect to green P-polarized light are sufficiently separated.Accordingly, green P-polarized light is not reflected by these filmsdespite shifting of the cutoff wavelengths due to incident angledependency. As a result, loss due to incident angle dependency does notoccur.

Alternatively, a configuration can be provided in which, for example,the cutoff wavelengths of the first film and the second film withrespect to green S-polarized light and red S-polarized light aresufficiently separated. Accordingly, the red and green S-polarized lightcan be synthesized by these films with virtually no loss despiteshifting of the cutoff wavelengths due to incident angle dependency.

Accordingly, colored light can be efficiently synthesized for light thatis entered at angles that differ from parallel light.

As described hereinabove, the present invention can provide anillumination device that can cause realization of the maximum opticaloutput performance of LEDs and that features high light utilizationefficiency during color mixing, and that can obtain white light havingsuperior white balance.

In addition, the present invention can provide a projection-type displaydevice in which this illumination device is used to obtain a brightprojected image.

The illumination device of each of the exemplary embodiments describedabove and the projection-type display device that uses the illuminationdevice are only examples of the present invention, and theconfigurations and operations of these devices are open to appropriatemodifications within a scope that does not diverge from the gist of thepresent invention.

For example, the illumination device of the first to third exemplaryembodiments, the illumination device of the fourth and fifth exemplaryembodiments, and the projection-type display device of the sixth andseventh exemplary embodiments can be combined as appropriate.

In addition, in the first to seventh exemplary embodiments, the relationbetween P-polarized light and S-polarized light can be reversed(description of P-polarized light can be described as S-polarized light,and description of S-polarized light can be described as P-polarizedlight).

Still further, the first and second dichroic mirrors are not limited todielectric multilayer films and may also be optical films havingwavelength selectivity or polarization selectivity such as holograms.

The angle of intersection of the first and second dichroic mirrors isnot limited to 90 degrees.

The first and second dichroic mirrors may be formed on plate glassrather than in prism shapes.

Still further, in the first to seventh exemplary embodiments, othersolid-state light sources such as semiconductor lasers may be used inplace of LEDs.

The display element is not limited to a component that usesmicromirrors. Instead of a reflective type, the display element may be atransmissive display element provided with a micro-shutter at eachpixel. The display element may also be a device other than a digitalmirror device, such as a liquid crystal light valve.

Although the present invention has been described with reference toexemplary embodiments, the present invention is not limited to theabove-described exemplary embodiments. The configuration and operationof the present invention is open to various modifications within a scopethat does not depart from the gist of the present invention that will beunderstood by one of ordinary skill in the art.

This application claims the benefits of priority based on JapanesePatent Application No. 2009-222671 for which application was submittedon Sep. 28, 2009 and incorporates by citation all of the disclosures ofthat application.

1. An illumination device comprising: a first light source that includesa solid-state light source having a peak wavelength that is set in a redwavelength band; a second light source that includes a solid-state lightsource having a peak wavelength that is set in a green wavelength band;a third light source that includes a solid-state light source having apeak wavelength that is set in a blue wavelength band; and a colorsynthesis optical element that synthesizes colored light of a firstpolarization that enters the color synthesis optical element from saidsecond light source and colored light of a second polarization having apolarization state that differs from a polarization state of said firstpolarization, the colored light of the second polarization entering thecolor synthesis optical element from said first light source and saidthird light source; wherein any one of said first light source, saidsecond light source and said third light source further includes atleast one solid-state light source having a peak wavelength that is setin a specific wavelength band that is a wavelength band of a color ofthe solid-state light source that is used in one of the remaining twolight sources, wherein said color synthesis optical element comprises:an exit surface; a first incident surface, a second incident surface anda third incident surface; and a first film and a second film providedsuch that a surface of the first film and a surface of the second filmintersect with each other, the first film and the second filmselectively reflecting or transmitting incident light according to thewavelength of the incident light, wherein: said first film transmits, ofvisible light of said first polarization, at least light of saidspecific wavelength band and reflects, of visible light of said secondpolarization, at least light of said specific wavelength band; saidsecond film transmits, of visible light of said first polarization, atleast light of said specific wavelength band and transmits, of saidvisible light of said second polarization, at least light of saidspecific wavelength band; and cutoff wavelengths of said first film andsaid second film with respect to said second polarization are set withinranges of bands other than the wavelength bands of red, green, and bluethat are three primary colors of light, and at least colored light ofsaid second polarization that enters the color synthesis optical elementfrom said first incident surface, colored light of said firstpolarization that enters the color synthesis optical element from saidsecond incident surface, and colored light of said second polarizationthat enters the color synthesis optical element from said third incidentsurface exit the color synthesis optical element from said exit surfaceby way of said first and second films, wherein the solid-state lightsource having the peak wavelength that is set in said specificwavelength band is a solid-state light source having a peak wavelengththat is set in said green wavelength band and is provided in said firstlight source.
 2. The illumination device as set forth in claim 1,wherein: the cutoff wavelengths with respect to said second polarizationof said first and second films are set within the range of theblue-green wavelength band; said first light source emits green light ofsaid second polarization whose peak wavelength is set in said greenwavelength band and red light of said second polarization whose peakwavelength is set in said red wavelength band; said second light sourceemits green light of said first polarization whose peak wavelength isset in said green wavelength band; and said third light source emitsblue light of said second polarization whose peak wavelength is set insaid blue wavelength band.
 3. The illumination device as set forth inclaim 2, wherein said second light source further includes a solid-statelight source having a peak wavelength that is set in a wavelength bandother than said green wavelength band.
 4. The illumination device as setforth in claim 3, wherein said solid-state light source whose peakwavelength is set in the wavelength band other than said greenwavelength band emits light of said first polarization.
 5. Theillumination device as set forth in claim 2, wherein said blue-greenwavelength band is at least 480 nm and no greater than 500 nm.
 6. Theillumination device as set forth in claim 1, further comprising fourright angle prisms including surfaces that form right angles, thesurfaces of each right angle prism being joined together, wherein saidfirst film and said second film are formed on the joined surfaces of theright angle prisms.
 7. The illumination device as set forth in claim 1,further comprising a retardation plate that converts light exited fromsaid color synthesis optical element to circularly polarized light.
 8. Aprojection-type display device comprising: the illumination device asset forth in claim 1; a display element into which light from saidillumination device is entered; projection optics that project an imagedisplayed by said display elements; and a control unit that both causesimages that accord with an input video signal to be displayed on saiddisplay element for each color component that corresponds to the threeprimary colors of light and that controls lighting of said first tothird light sources that make up said illumination device insynchronization with the timing of the image display of each colorcomponent.